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/core/services/mpv.ts

@@ -123,7 +123,6 @@ export interface MpvIpcClientDeps extends MpvIpcClientProtocolDeps {}
 export interface MpvIpcClientEventMap {
   'connection-change': { connected: boolean };
   'subtitle-change': { text: string; isOverlayVisible: boolean };
-  'subtitle-ass-change': { text: string };
   'subtitle-timing': { text: string; start: number; end: number };
   'time-pos-change': { time: number };
   'duration-change': { duration: number };
@@ -417,9 +416,6 @@ export class MpvIpcClient implements MpvClient {
       emitSubtitleChange: (payload) => {
         this.emit('subtitle-change', payload);
       },
-      emitSubtitleAssChange: (payload) => {
-        this.emit('subtitle-ass-change', payload);
-      },
       emitSubtitleTiming: (payload) => {
         this.emit('subtitle-timing', payload);
       },