Add SDL3-based controller input service

Integrate SDL3 controller support and wire it into the InputRouter.

- Add ControllerInputService (src/ControllerInputService.{h,cpp}) to discover, poll and translate SDL3 gamepad events into InputRouter actions, with axis repeat handling and debouncing.
- Update CMakeLists to find or fetch SDL3, add the new source files to the target, link the SDL3 target, and copy runtime DLLs on Windows.
- Add triggerAction(Action) to InputRouter and use it from existing keyboard handling to centralize action dispatch.
- Instantiate ControllerInputService in main so controllers feed the InputRouter.
- Update QML views (ShellWindow, HomeView, LibraryView, SettingsView) to use numeric action IDs and add small UI/status text and navigation tweaks for controller-driven flows.

These changes enable gamepad/controller input for Bigscreen via SDL3 and adapt UI code to handle the mapped actions.

Bigscreen/qml/ShellWindow.qml

@@ -70,7 +70,7 @@ ApplicationWindow {
                 window.handlePowerInput(action)
                 return
             }
-            if (action === InputRouter.Menu) {
+            if (action === 7) {
                 window.powerOverlayVisible = true
                 return
             }
@@ -120,16 +120,16 @@ ApplicationWindow {
 
     function handlePowerInput(action) {
         switch (action) {
-        case InputRouter.Up:
+        case 1:
             powerOverlay.moveFocus(-1)
             break
-        case InputRouter.Down:
+        case 2:
             powerOverlay.moveFocus(1)
             break
-        case InputRouter.Accept:
+        case 5:
             powerOverlay.activateFocused()
             break
-        case InputRouter.Back:
+        case 6:
             window.powerOverlayVisible = false
             break
         default: