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/static/locales/en.json

@@ -101,6 +101,11 @@
   "devices.wled_webui_link": "WLED Web UI",
   "devices.wled_note_webui": "(open your device's IP in a browser).",
   "devices.wled_note2": "This controller sends pixel color data and controls brightness per device.",
+  "device.scan": "Auto Discovery",
+  "device.scan.empty": "No WLED devices found on the network",
+  "device.scan.error": "Network scan failed",
+  "device.scan.already_added": "Already added",
+  "device.scan.selected": "Device selected",
   "device.type": "Device Type:",
   "device.type.hint": "Select the type of LED controller",
   "device.url.hint": "IP address or hostname of the device (e.g. http://192.168.1.100)",