feat: add HLS cache toggle and cache management to Settings

Chromium's HTTP disk cache accumulates HLS segments indefinitely on
Steam Deck — single-use segments have ~0% cache-hit rate but cause
unbounded SSD writes and disk growth. This wires up a runtime toggle
(default: cache disabled) so the user can control the behaviour without
restarting the app.

- Introduce src/main/prefs/hlsCachePref.ts as a shared mutable so
  index.ts and handlers.ts don't form a circular import
- Restructure onHeadersReceived in setupTwitchCors: CORS-bypass and
  no-store now run as independent blocks; no-store fires only for HLS
  extensions (.m3u8/.ts/.m4s/.aac, path before ?) when the toggle is off
- Add IPC handlers app:get-cache-size, app:clear-cache,
  app:set-hls-cache-enabled; wire through preload bridge and t4sd.d.ts
- Extend SettingsContext with hlsCacheEnabled (default false); push
  persisted value to main on mount and on every change
- Add "Speicher" section to SettingsScreen with HLS toggle row and
  cache-size display + clear button; gamepad-navigable

src/main/prefs/hlsCachePref.ts

@@ -0,0 +1,9 @@
+let hlsCacheEnabled = false
+
+export function getHlsCacheEnabled(): boolean {
+  return hlsCacheEnabled
+}
+
+export function setHlsCacheEnabled(enabled: boolean): void {
+  hlsCacheEnabled = enabled
+}