Add WLED auto-discovery via mDNS with zeroconf

Scan the local network for WLED devices advertising _wled._tcp.local.
and present them in the Add Device modal for one-click selection.

- New discovery.py: async mDNS browse + parallel /json/info enrichment
- GET /api/v1/devices/discover endpoint with already_added dedup
- Header scan button (magnifying glass icon) in add-device modal
- Discovered devices show name, IP, LED count, version; click to fill form
- en/ru locale strings for discovery UI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/schemas/devices.py

@@ -113,3 +113,24 @@ class DeviceStateResponse(BaseModel):
     device_error: Optional[str] = Field(None, description="Last health check error")
     test_mode: bool = Field(default=False, description="Whether calibration test mode is active")
     test_mode_edges: List[str] = Field(default_factory=list, description="Currently lit edges in test mode")
+
+
+class DiscoveredDeviceResponse(BaseModel):
+    """A single device found via network discovery."""
+
+    name: str = Field(description="Device name (from mDNS or firmware)")
+    url: str = Field(description="Device URL")
+    device_type: str = Field(default="wled", description="Device type")
+    ip: str = Field(description="IP address")
+    mac: str = Field(default="", description="MAC address")
+    led_count: Optional[int] = Field(None, description="LED count (if reachable)")
+    version: Optional[str] = Field(None, description="Firmware version")
+    already_added: bool = Field(default=False, description="Whether this device is already in the system")
+
+
+class DiscoverDevicesResponse(BaseModel):
+    """Response from device discovery scan."""
+
+    devices: List[DiscoveredDeviceResponse] = Field(description="Discovered devices")
+    count: int = Field(description="Total devices found")
+    scan_duration_ms: float = Field(description="How long the scan took in milliseconds")