feat(dashboard): per-account customizable dashboard with slide-in panel

Open-registry section/perf-cell schema persisted server-side under
db.get_setting('dashboard_layout'); localStorage cache for instant
first-paint, server sync after auth. 5 built-in presets
(Studio/Operator/Showrunner/Diagnostics/TV); JSON export/import.

Slide-in Customize panel toggles section + perf-cell visibility,
reorders via hand-rolled HTML5 drag (with up/down buttons for
keyboard/TV-remote use), changes density per section, and exposes
global Width / Animations / Perf-mode / Window with per-cell Inherit
overrides.

Window setting now drives the actual sparkline slice (30s/1m/2m/5m at
configurable poll interval) instead of always rendering 120 fixed
samples. Perf-grid edits re-render in place — sparklines repaint from
persistent module-level history, value labels replay from cached
last-fetch payload, so there is no flicker frame and no zero-data
window between layout change and next poll. initPerfCharts now fires
an immediate fetch on init so reload no longer shows "—" until the
first interval tick.

Reset confirmation uses the project's themed showConfirm modal
instead of the browser dialog. Reserved registry keys (audio-meters,
alerts, led-preview, source-thumbs, pinned, flow) are forward-
compatible so v1.1 cards slot in without a schema bump.

Backend exposes GET/PUT/DELETE /api/v1/preferences/dashboard-layout
treating the body as opaque JSON with a numeric version gate; covered
by 6 round-trip / validation / unknown-field tests.

TODO.md

@@ -171,6 +171,52 @@ Phases are independent and CSS-only where possible — backend untouched.
       as a separate cleanup PR after the new design has soaked.
 - [WONTDO] Delete `mobile.css` — Phase 6 kept the filename.
 
+## Dashboard Customization
+
+Per-account dashboard layout — slide-in Customize panel lets users
+toggle section / perf-cell visibility, reorder via drag, change density,
+pick presets, and import/export the layout as JSON. Server-synced via
+`db.get_setting('dashboard_layout')` so settings follow the user.
+
+- [x] `js/features/dashboard-layout.ts` — schema (open registry of section
+      / perf-cell keys so v1.1 cards slot in with no migration), defaults,
+      5 built-in presets (Studio/Operator/Showrunner/Diagnostics/TV),
+      localStorage cache + server sync, legacy-key migration from
+      `dashboard_collapsed`, `perfMetricsMode`, `perfChartColor_*`.
+- [x] `api/routes/preferences.py` — `GET/PUT/DELETE
+      /api/v1/preferences/dashboard-layout`. Treats payload as opaque
+      (frontend owns the schema); validates only that body is an object
+      with a numeric `version`. 6 pytest tests in
+      `tests/test_preferences_api.py` cover round-trip, default-empty,
+      validation, delete, and unknown-field passthrough.
+- [x] `js/features/dashboard.ts` — sections rendered into a fragment map,
+      then assembled in layout-driven order; perf section stays pinned
+      top (chart-persistence reasons) but its visibility is layout-
+      driven. Layout-change subscription invalidates the in-place-update
+      optimization so density / order / visibility changes always
+      rebuild section HTML.
+- [x] `js/features/perf-charts.ts` — `renderPerfSection()` iterates
+      `getOrderedPerfCells()`; existing legacy `setPerfMode` writes
+      through to the layout so the global toggle and the customize
+      panel stay in sync.
+- [x] `js/features/dashboard-customize.ts` + `css/dashboard-customize.css`
+      — slide-in panel, hand-rolled HTML5 drag-and-drop reorder, ↑/↓
+      buttons for keyboard / TV remote, debounced (300 ms) autosave,
+      live preview while open. Reset / export / import actions.
+- [x] i18n keys for `dashboard.customize.*` in en/ru/zh.
+- [ ] (v1.1) Audio meters section — peak / RMS / BPM bars per audio
+      source. Schema key `audio-meters` already reserved.
+- [ ] (v1.1) Alerts section — quiet by default, loud on issues.
+      Reserved key `alerts`.
+- [ ] (v1.1) Live LED preview strip per running device. Reserved
+      key `led-preview`.
+- [ ] (v1.1) Source thumbnails grid (1 fps multiviewer). Reserved
+      key `source-thumbs`.
+- [ ] (v1.2) Pinned section (user-curated mix of targets / scenes /
+      devices). Reserved key `pinned`.
+- [ ] (v1.2) Patch/flow map — read-only mini graph of routing.
+      Reserved key `flow`.
+
 ## BLE LED Controller Support (SP110E / Triones / Zengge / Govee)
 
 Add support for Bluetooth Low Energy LED controllers driven by mobile apps like "LED Hue", HappyLighting, iLightsIn. Whole-strip ambient-color output only — these protocols don't support per-pixel streaming.