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/views/HomeView.qml

@@ -39,15 +39,18 @@ ColumnLayout {
 
     function handleInput(action) {
         switch (action) {
-        case InputRouter.Left:
+        case 3:
             rail.moveFocus(-1)
             break
-        case InputRouter.Right:
+        case 4:
             rail.moveFocus(1)
             break
-        case InputRouter.Accept:
+        case 5:
             rail.activateFocused()
             break
+        case 6:
+            root.navigate("power")
+            break
         default:
             break
         }