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/src/wled_controller/api/routes/templates.py

@@ -215,22 +215,23 @@ async def delete_template(
 
 @router.get("/api/v1/capture-engines", response_model=EngineListResponse, tags=["Templates"])
 async def list_engines(_auth: AuthRequired):
-    """List available capture engines on this system.
+    """List all registered capture engines.
 
-    Returns all registered engines that are available on the current platform.
+    Returns every registered engine with an ``available`` flag showing
+    whether it can be used on the current system.
     """
     try:
-        available_engine_types = EngineRegistry.get_available_engines()
+        available_set = set(EngineRegistry.get_available_engines())
+        all_engines = EngineRegistry.get_all_engines()
 
         engines = []
-        for engine_type in available_engine_types:
-            engine_class = EngineRegistry.get_engine(engine_type)
+        for engine_type, engine_class in all_engines.items():
             engines.append(
                 EngineInfo(
                     type=engine_type,
                     name=engine_type.upper(),
                     default_config=engine_class.get_default_config(),
-                    available=True,
+                    available=(engine_type in available_set),
                 )
             )
 
@@ -242,7 +243,7 @@ async def list_engines(_auth: AuthRequired):
 
 
 @router.post("/api/v1/capture-templates/test", response_model=TemplateTestResponse, tags=["Templates"])
-async def test_template(
+def test_template(
     test_request: TemplateTestRequest,
     _auth: AuthRequired,
     processor_manager: ProcessorManager = Depends(get_processor_manager),
@@ -250,6 +251,10 @@ async def test_template(
 ):
     """Test a capture template configuration.
 
+    Uses sync ``def`` so FastAPI runs it in a thread pool — the engine
+    initialisation and capture loop are blocking and would stall the
+    event loop if run in an ``async def`` handler.
+
     Temporarily instantiates an engine with the provided configuration,
     captures frames for the specified duration, and returns actual FPS metrics.
     """
@@ -301,8 +306,9 @@ async def test_template(
             screen_capture = stream.capture_frame()
             capture_elapsed = time.perf_counter() - capture_start
 
-            # Skip if no new frame (screen unchanged)
+            # Skip if no new frame (screen unchanged); yield CPU
             if screen_capture is None:
+                time.sleep(0.005)
                 continue
 
             total_capture_time += capture_elapsed