Comprehensive WebUI review: 41 UX/feature/CSS improvements

Safety & Correctness:
- Add confirmation dialogs to Stop All, turnOffDevice
- i18n confirm dialog (title, yes, no buttons)
- Fix duplicate tutorial-overlay ID
- Define missing CSS variables (--radius, --text-primary, --hover-bg, --input-bg)
- Fix toast z-index conflict with confirm dialog (2500 → 3000)

UX Consistency:
- Add backdrop-close to test modals
- Add device clone feature (only entity without it)
- Add sync clocks to command palette
- Replace 20+ hardcoded accent colors with CSS vars/color-mix()
- Remove dead .badge duplicate from components.css
- Make calibration elements keyboard-accessible (div → button)
- Add aria-labels to color picker swatches
- Fix pattern canvas mobile horizontal scroll
- Fix graph editor mobile bottom clipping

Polish:
- Add empty-state messages to all CardSection instances
- Convert 21 px font-sizes to rem
- Add scroll-behavior: smooth with reduced-motion override
- Add @media print styles
- Add :focus-visible to 4 missing interactive elements
- Fix settings modal close label (Cancel → Close)
- Fix api-key submit button i18n

New Features:
- Command palette actions: start/stop targets, activate scenes, enable/disable
- Bulk start/stop API endpoints (POST /output-targets/bulk/start|stop)
- OS notification history viewer modal
- Scene "used by" automation reference count on cards
- Clock elapsed time display on Streams tab cards
- Device "last seen" relative timestamp on cards
- Audio device refresh button in edit modal
- Composite layer drag-to-reorder
- MQTT settings panel (broker config with JSON persistence)
- WebSocket log viewer with level filtering and ring buffer
- Runtime log-level adjustment (GET/PUT endpoints + settings UI)
- Animated value source waveform canvas preview
- Gradient custom preset save/delete (localStorage)
- API key read-only display in settings
- Backup metadata (file size, auto/manual badges)
- Server restart button with confirm + overlay
- Partial config export/import per entity type
- Progressive disclosure in target editor (Advanced section)

CSS Architecture:
- Define radius scale tokens (--radius-sm/md/lg/pill)
- Scope .cs-filter selectors to remove 7 !important overrides
- Consolidate duplicate toggle switch (filter-list → settings-toggle)

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

server/src/wled_controller/templates/modals/css-editor.html

@@ -109,7 +109,7 @@
                             <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
                         </div>
                         <small class="input-hint" style="display:none" data-i18n="color_strip.gradient.preset.hint">Load a predefined gradient palette. Selecting a preset replaces the current stops.</small>
-                        <select id="css-editor-gradient-preset" onchange="applyGradientPreset(this.value)">
+                        <select id="css-editor-gradient-preset" onchange="onGradientPresetChange(this.value)">
                             <option value="" data-i18n="color_strip.gradient.preset.custom">— Custom —</option>
                             <option value="rainbow" data-i18n="color_strip.gradient.preset.rainbow">Rainbow</option>
                             <option value="sunset" data-i18n="color_strip.gradient.preset.sunset">Sunset</option>
@@ -124,6 +124,12 @@
                             <option value="neon" data-i18n="color_strip.gradient.preset.neon">Neon</option>
                             <option value="pastel" data-i18n="color_strip.gradient.preset.pastel">Pastel</option>
                         </select>
+                        <div style="margin-top:6px;">
+                            <button type="button" class="btn btn-secondary btn-sm"
+                                onclick="promptAndSaveGradientPreset()"
+                                data-i18n="color_strip.gradient.preset.save_button">Save as preset…</button>
+                        </div>
+                        <div id="css-editor-custom-presets-list" class="custom-presets-list"></div>
                     </div>
 
                     <div class="form-group">