refactor(ipc): remove dead preload methods and the ASS line broadcast (#293)

Drop 16 overlay preload methods with no caller (onSubtitleAss,
getCurrentSubtitleAss, onVisibility, getOverlayVisibility,
openYomitanSettings, quitApp, toggleOverlay, getMecabStatus,
setMecabEnabled, getKeybindings, saveControllerPreference,
getAnkiConnectStatus, setAnkiConnectEnabled, clearAnkiConnectHistory,
cycleRuntimeOption, appendClipboardVideoToQueue) along with their
IPC_CHANNELS keys, handlers, validators and dep plumbing.

With both ASS consumers gone, stop observing sub-text/ass in mpv and
remove the subtitle-ass-change event chain and appState.currentSubAssText.
The main-process functions these wrapped stay; tray, CLI, shortcuts and
session actions still use them.

src/renderer/handlers/keyboard.test.ts

@@ -242,7 +242,6 @@ function installKeyboardTestGlobals() {
         windowFocusCalls += 1;
       },
       electronAPI: {
-        getKeybindings: async () => [],
         getMpvInputBindings: () => getMpvInputBindings(),
         getSessionBindings: () => getSessionBindingsImpl(),
         getConfiguredShortcuts: async () => configuredShortcuts,