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/main/runtime/mpv-main-event-bindings.test.ts

@@ -29,8 +29,6 @@ function createDeps(overrides: Partial<Deps> = {}): Deps {
     onSubtitleChange: () => {},
     refreshDiscordPresence: () => {},
 
-    setCurrentSubAssText: () => {},
-    broadcastSubtitleAss: () => {},
     broadcastSecondarySubtitle: () => {},
 
     updateCurrentMediaPath: () => {},