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/static/js/features/css-gradient-editor.js

@@ -378,6 +378,41 @@ function _gradientStartDrag(e, idx) {
     document.addEventListener('mouseup', onUp);
 }
 
+/* ── Custom presets (localStorage) ───────────────────────────── */
+
+const _CUSTOM_PRESETS_KEY = 'custom_gradient_presets';
+
+/** Load custom presets from localStorage. Returns an array of { name, stops }. */
+export function loadCustomGradientPresets() {
+    try {
+        return JSON.parse(localStorage.getItem(_CUSTOM_PRESETS_KEY) || '[]');
+    } catch {
+        return [];
+    }
+}
+
+/** Save the current gradient stops as a named custom preset. */
+export function saveCurrentAsCustomPreset(name) {
+    if (!name) return;
+    const stops = _gradientStops.map(s => ({
+        position: s.position,
+        color: [...s.color],
+        ...(s.colorRight ? { color_right: [...s.colorRight] } : {}),
+    }));
+    const presets = loadCustomGradientPresets();
+    // Replace if same name exists
+    const idx = presets.findIndex(p => p.name === name);
+    if (idx >= 0) presets[idx] = { name, stops };
+    else presets.push({ name, stops });
+    localStorage.setItem(_CUSTOM_PRESETS_KEY, JSON.stringify(presets));
+}
+
+/** Delete a custom preset by name. */
+export function deleteCustomGradientPreset(name) {
+    const presets = loadCustomGradientPresets().filter(p => p.name !== name);
+    localStorage.setItem(_CUSTOM_PRESETS_KEY, JSON.stringify(presets));
+}
+
 /* ── Track click → add stop ───────────────────────────────────── */
 
 function _gradientSetupTrackClick() {