feat: expand keybinding system with 4 new customizable actions

- Add World Map (W), Nameplates (V), Raid Frames (R), Quest Log (Q) to
  KeybindingManager enum with customizable default bindings
- Replace hard-coded V key check for nameplate toggle with
  KeybindingManager::isActionPressed() to support customization
- Update config file persistence to handle new bindings
- Infrastructure in place for implementing visibility toggles on other
  windows (World Map, Raid Frames, Quest Log) with future UI refactoring

src/ui/game_screen.cpp

@@ -1470,8 +1470,8 @@ void GameScreen::processTargetInput(game::GameHandler& gameHandler) {
             }
         }
 
-        // V — toggle nameplates (WoW default keybinding)
-        if (input.isKeyJustPressed(SDL_SCANCODE_V)) {
+        // Toggle nameplates (customizable keybinding, default V)
+        if (KeybindingManager::getInstance().isActionPressed(KeybindingManager::Action::TOGGLE_NAMEPLATES)) {
             showNameplates_ = !showNameplates_;
         }