Add demo mode: virtual hardware sandbox for testing without real devices

Demo mode provides a complete sandbox environment with:
- Virtual capture engine (radial rainbow test pattern on 3 displays)
- Virtual audio engine (synthetic music-like audio on 2 devices)
- Virtual LED device provider (strip/60, matrix/256, ring/24 LEDs)
- Isolated data directory (data/demo/) with auto-seeded sample entities
- Dedicated config (config/demo_config.yaml) with pre-configured API key
- Frontend indicator (DEMO badge + dismissible banner)
- Engine filtering (only demo engines visible in demo mode)
- Separate entry point: python -m wled_controller.demo (port 8081)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

TODO.md

@@ -1,129 +0,0 @@
-# Pending Features & Issues
-
-Priority: `P1` quick win · `P2` moderate · `P3` large effort
-
-## Processing Pipeline
-
-- [ ] `P3` **Transition effects** — Crossfade, wipe, or dissolve between sources/profiles instead of instant cut
-  - Complexity: large — requires a new transition layer concept in ProcessorManager; must blend two live streams simultaneously during switch, coordinating start/stop timing
-  - Impact: medium — polishes profile switching UX but ambient lighting rarely switches sources frequently
-
-## Output Targets
-
-- [ ] `P2` **Art-Net / sACN (E1.31)** — Stage/theatrical lighting protocols, DMX controllers
-  - Complexity: medium — UDP-based protocols with well-documented specs; similar architecture to DDP client; needs DMX universe/channel mapping UI
-  - Impact: medium — opens stage/theatrical use case, niche but differentiating
-
-## Capture Engines
-
-- [ ] `P3` **SCRCPY capture engine** — Implement SCRCPY-based screen capture for Android devices
-  - Complexity: large — external dependency on scrcpy binary; need to manage subprocess lifecycle, parse video stream (ffmpeg/AV pipe), handle device connect/disconnect
-  - Impact: medium — enables phone screen mirroring to ambient lighting; appeals to mobile gaming use case
-
-## Code Health
-
-- [ ] `P1` **"Start All" targets button** — "Stop All" exists but "Start All" is missing
-- [x] `P2` **Manual backup trigger endpoint** — `POST /system/auto-backup/trigger` (~5 lines)
-- [ ] `P2` **Scene snapshot should capture device brightness** — `software_brightness` not saved/restored
-- [ ] `P2` **Distinguish "crashed" vs "stopped" in dashboard** — `metrics.last_error` is already populated
-- [ ] `P3` **CSS source import/export** — share individual sources without full config backup
-
-## Backend Review Fixes (2026-03-14)
-
-### Performance
-- [x] **P1** PIL blocking in async handlers → `asyncio.to_thread`
-- [x] **P2** `subprocess.run` blocking event loop → `asyncio.create_subprocess_exec`
-- [x] **P3** Audio enum blocking async → `asyncio.to_thread`
-- [x] **P4** Display enum blocking async → `asyncio.to_thread`
-- [x] **P5** `colorsys` scalar loop in hot path → vectorize numpy
-- [x] **P6** `MappedStream` per-frame allocation → double-buffer
-- [x] **P7** Audio/effect per-frame temp allocs → pre-allocate
-- [x] **P8** Blocking `httpx.get` in stream init → documented (callers use to_thread)
-- [x] **P9** No-cache middleware runs on all requests → scope to static
-- [x] **P10** Sync file I/O in async handlers (stores) → documented as accepted risk (< 5ms)
-- [x] **P11** `frame_time` float division every loop iter → cache field
-- [x] **P12** `_check_name_unique` O(N) + no lock → add threading.Lock
-- [x] **P13** Imports inside 1-Hz metrics loop → move to module level
-
-### Code Quality
-
-- [x] **Q1** `DeviceStore` not using `BaseJsonStore`
-- [x] **Q2** `ColorStripStore` 275-line god methods → factory dispatch
-- [x] **Q3** Layer violation: core imports from routes → extract to utility
-- [x] **Q4** 20+ field-by-field update in Device/routes → dataclass + generic update
-- [x] **Q5** WebSocket auth copy-pasted 9x → extract helper
-- [x] **Q6** `set_device_brightness` bypasses store → use update_device
-- [x] **Q7** DI via 16+ module globals → registry pattern
-- [x] **Q8** `_css_to_response` 30+ getattr → polymorphic to_response
-- [x] **Q9** Private attribute access across modules → expose as properties
-- [x] **Q10** `ColorStripSource.to_dict()` emits ~25 nulls → per-subclass override
-- [x] **Q11** `DeviceStore.get_device` returns None vs raises → raise ValueError
-- [x] **Q12** `list_all_tags` fragile method-name probing → use get_all()
-- [x] **Q13** Route create/update pass 30 individual fields → **kwargs
-
-## UX
-
-- [ ] `P1` **Collapse dashboard running target stats** — Show only FPS chart by default; uptime, errors, and pipeline timings in an expandable section collapsed by default
-- [x] `P1` **Daylight brightness value source** — New value source type that reports a 0–255 brightness level based on daylight cycle time (real-time or simulated), reusing the daylight LUT logic
-- [x] `P1` **Tags input: move under name, remove hint/title** — Move the tags chip input directly below the name field in all entity editor modals; remove the hint toggle and section title for a cleaner layout
-
-## WebUI Review (2026-03-16)
-
-### Critical (Safety & Correctness)
-- [x] `P1` **"Stop All" buttons need confirmation** — dashboard, LED targets, KC targets
-- [x] `P1` **`turnOffDevice()` needs confirmation**
-- [x] `P1` **Confirm dialog i18n** — added data-i18n to title/buttons
-- [x] `P1` **Duplicate `id="tutorial-overlay"`** — renamed to calibration-tutorial-overlay
-- [x] `P1` **Define missing CSS variables** — --radius, --text-primary, --hover-bg, --input-bg
-- [x] `P1` **Toast z-index conflict** — toast now 3000
-
-### UX Consistency
-- [x] `P1` **Test modals backdrop-close** — added setupBackdropClose
-- [x] `P1` **Devices clone** — added cloneDevice with full field prefill
-- [x] `P1` **Sync clocks in command palette** — added to _responseKeys + _buildItems
-- [x] `P2` **Hardcoded accent colors** — 20+ replacements using color-mix() and CSS vars
-- [x] `P2` **Duplicate `.badge` definition** — removed dead code from components.css
-- [x] `P2` **Calibration elements keyboard-accessible** — changed div to button
-- [x] `P2` **Color-picker swatch aria-labels** — added aria-label with hex value
-- [x] `P2` **Pattern canvas mobile scroll** — added min-width: 0 override in mobile.css
-- [x] `P2` **Graph editor mobile bottom clipping** — adjusted height in mobile.css
-
-### Low Priority Polish
-- [x] `P3` **Empty-state illustrations/onboarding** — CardSection emptyKey with per-entity messages
-- [x] `P3` **api-key-modal submit title i18n**
-- [x] `P3` **Settings modal close labeled "Cancel" → "Close"**
-- [x] `P3` **Inconsistent px vs rem font sizes** — 21 conversions across streams/modal/cards CSS
-- [x] `P3` **scroll-behavior: smooth** — added with reduced-motion override
-- [x] `P3` **Reduce !important usage** — scoped .cs-filter selectors
-- [x] `P3` **@media print styles** — theme reset + hide nav
-- [x] `P3` **:focus-visible on interactive elements** — added 4 missing selectors
-- [x] `P3` **iOS Safari modal scroll-position jump** — already implemented in ui.js lockBody/unlockBody
-
-### New Features
-- [x] `P1` **Command palette actions** — start/stop targets, activate scenes, enable/disable automations
-- [x] `P1` **Bulk start/stop API** — POST /output-targets/bulk/start and /bulk/stop
-- [x] `P1` **OS notification history viewer** — modal with app name, timestamp, fired/filtered badges
-- [x] `P1` **Scene "used by" reference count** — badge on card with automation count
-- [x] `P1` **Clock elapsed time on cards** — shows formatted elapsed time
-- [x] `P1` **Device "last seen" timestamp** — relative time with full ISO in title
-- [x] `P2` **Audio device refresh in modal** — refresh button next to device dropdown
-- [x] `P2` **Composite layer reorder** — drag handles with pointer-based reorder
-- [x] `P2` **MQTT settings panel** — config form with enabled/host/port/auth/topic, JSON persistence
-- [x] `P2` **Log viewer** — WebSocket broadcaster with ring buffer, level-filtered UI in settings
-- [x] `P2` **Animated value source waveform preview** — canvas drawing of sine/triangle/sawtooth/square
-- [x] `P2` **Gradient custom preset save** — localStorage-backed custom presets with save/delete
-- [x] `P2` **API key management UI** — read-only display of key labels with masked values
-- [x] `P2` **Backup metadata** — file size, auto/manual badge
-- [x] `P2` **Server restart button** — in settings with confirm dialog + restart overlay
-- [x] `P2` **Partial config export/import** — per-store export/import with merge option
-- [x] `P3` **Audio spectrum visualizer** — already fully implemented
-- [ ] `P3` **Hue bridge pairing flow** — requires physical Hue bridge hardware
-- [x] `P3` **Runtime log-level adjustment** — GET/PUT endpoints + settings dropdown
-- [x] `P3` **Progressive disclosure in target editor** — advanced section collapsed by default
-
-### CSS Architecture
-- [x] `P1` **Define missing CSS variables** — --radius, --text-primary, --hover-bg, --input-bg
-- [x] `P2` **Define radius scale** — --radius-sm/md/lg/pill tokens, migrated key selectors
-- [x] `P2` **Scope generic input selector** — .cs-filter boosted specificity, 7 !important removed
-- [x] `P2` **Consolidate duplicate toggle switch** — filter-list uses settings-toggle
-- [x] `P2` **Replace hardcoded accent colors** — 20+ values → CSS vars with color-mix()