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/api/schemas/system.py

@@ -115,3 +115,43 @@ class BackupListResponse(BaseModel):
 
     backups: List[BackupFileInfo]
     count: int
+
+
+# ─── MQTT schemas ──────────────────────────────────────────────
+
+class MQTTSettingsResponse(BaseModel):
+    """MQTT broker settings response (password is masked)."""
+
+    enabled: bool = Field(description="Whether MQTT is enabled")
+    broker_host: str = Field(description="MQTT broker hostname or IP")
+    broker_port: int = Field(ge=1, le=65535, description="MQTT broker port")
+    username: str = Field(description="MQTT username (empty = anonymous)")
+    password_set: bool = Field(description="Whether a password is configured")
+    client_id: str = Field(description="MQTT client ID")
+    base_topic: str = Field(description="Base topic prefix")
+
+
+class MQTTSettingsRequest(BaseModel):
+    """MQTT broker settings update request."""
+
+    enabled: bool = Field(description="Whether MQTT is enabled")
+    broker_host: str = Field(description="MQTT broker hostname or IP")
+    broker_port: int = Field(ge=1, le=65535, description="MQTT broker port")
+    username: str = Field(default="", description="MQTT username (empty = anonymous)")
+    password: str = Field(default="", description="MQTT password (empty = keep existing if omitted)")
+    client_id: str = Field(default="ledgrab", description="MQTT client ID")
+    base_topic: str = Field(default="ledgrab", description="Base topic prefix")
+
+
+# ─── Log level schemas ─────────────────────────────────────────
+
+class LogLevelResponse(BaseModel):
+    """Current log level response."""
+
+    level: str = Field(description="Current effective log level name (e.g. DEBUG, INFO, WARNING, ERROR, CRITICAL)")
+
+
+class LogLevelRequest(BaseModel):
+    """Request to change the log level."""
+
+    level: str = Field(description="New log level name (DEBUG, INFO, WARNING, ERROR, CRITICAL)")