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/LibraryView.qml

@@ -14,6 +14,7 @@ ColumnLayout {
     ]
 
     property int focusIndex: 0
+    property string statusText: "Mock entries for v0 — scanners and launchers come later."
 
     spacing: 20
 
@@ -25,7 +26,7 @@ ColumnLayout {
     }
 
     Text {
-        text: "Mock entries for v0 — scanners and launchers come later."
+        text: root.statusText
         font: Theme.metaFont
         color: Theme.textMuted
         Layout.leftMargin: 40
@@ -79,18 +80,19 @@ ColumnLayout {
 
     function handleInput(action) {
         switch (action) {
-        case InputRouter.Up:
+        case 1:
             focusIndex = Math.max(0, focusIndex - 1)
             list.currentIndex = focusIndex
             break
-        case InputRouter.Down:
+        case 2:
             focusIndex = Math.min(entries.length - 1, focusIndex + 1)
             list.currentIndex = focusIndex
             break
-        case InputRouter.Back:
+        case 6:
             root.goBack()
             break
-        case InputRouter.Accept:
+        case 5:
+            statusText = entries[focusIndex].title + " selected — launcher integration comes later."
             break
         default:
             break