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/pyproject.toml

@@ -37,6 +37,7 @@ dependencies = [
     "python-dateutil>=2.9.0",
     "python-multipart>=0.0.12",
     "wmi>=1.5.1; sys_platform == 'win32'",
+    "zeroconf>=0.131.0",
 ]
 
 [project.optional-dependencies]