feat(embedded-mpv): Settings toggle for the frame-copy engine

New 'Embedded MPV: frame-copy engine' checkbox in Settings > Playback,
shown only when the machine can run it (macOS arm64 with the helper binary
present — support now reports frameCopyAvailable). The choice persists to
the main-process config store because the engine relaxes the window sandbox
for the preload frame pump, which is fixed at window creation: main.ts
reads the store before creating the window and sets the engine env var; an
explicitly set env var (including '0') always wins, and the UI shows a
restart hint while the saved choice differs from the active engine.
Localized in all 18 locales.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/embedded-mpv-session.interface.ts

@@ -35,6 +35,12 @@ export interface EmbeddedMpvSupport {
      * `frame-copy` = helper process + shm ring + renderer canvas.
      */
     engine?: EmbeddedMpvEngine;
+    /**
+     * True when this machine could run the frame-copy engine (macOS arm64
+     * with the helper binary present), regardless of whether it is active.
+     * Drives the Settings toggle; switching engines requires an app restart.
+     */
+    frameCopyAvailable?: boolean;
 }
 
 /**

libs/shared/interfaces/src/lib/settings.interface.ts

@@ -108,6 +108,12 @@ export interface Settings {
     downloadFolder?: string;
     /** Custom live recording folder path (uses system Downloads folder if not set) */
     recordingFolder?: string;
+    /**
+     * Embedded MPV frame-copy engine (experimental, macOS Apple Silicon).
+     * Applied on the next app start — the engine relaxes the window sandbox
+     * for its preload frame pump, which is fixed at window creation.
+     */
+    embeddedMpvFrameCopy?: boolean;
     /** Cover/poster sizing preset applied across grids and rails */
     coverSize?: CoverSize;
     /** Live EPG panel layout: horizontal timeline (default) or vertical list */