Add ADB-based Android screen capture engine with display picker integration

New scrcpy/ADB capture engine that captures Android device screens over
ADB using screencap polling. Supports USB and WiFi ADB connections with
device auto-discovery. Engine-aware display picker shows Android devices
when scrcpy engine is selected, with inline ADB connect form for WiFi
devices.

Key changes:
- New scrcpy_engine.py using adb screencap polling (~1-2 FPS over WiFi)
- Engine-aware GET /config/displays?engine_type= API
- ADB connect/disconnect API endpoints (POST /adb/connect, /adb/disconnect)
- Display picker supports engine-specific device lists
- Stream/test modals pass engine type to display picker
- Test template handler changed to sync def to prevent event loop blocking
- Restart script merges registry PATH for newly-installed tools
- All engines (including unavailable) shown in engine list with status flag

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

server/restart.ps1

@@ -8,6 +8,17 @@ foreach ($p in $procs) {
 }
 if ($procs) { Start-Sleep -Seconds 2 }
 
+# Merge registry PATH with current PATH so newly-installed tools (e.g. scrcpy) are visible
+$regUser = [Environment]::GetEnvironmentVariable('PATH', 'User')
+if ($regUser) {
+    $currentDirs = $env:PATH -split ';' | ForEach-Object { $_.TrimEnd('\') }
+    foreach ($dir in ($regUser -split ';')) {
+        if ($dir -and ($currentDirs -notcontains $dir.TrimEnd('\'))) {
+            $env:PATH = "$env:PATH;$dir"
+        }
+    }
+}
+
 # Start server detached
 Write-Host "Starting server..."
 Start-Process -FilePath python -ArgumentList '-m', 'wled_controller.main' `