feat(overlay): add loading OSD spinner and queue notifications until ren

- Show mpv OSD spinner from start-file until subminer-overlay-loading-ready; force-shown for visible-overlay startup regardless of osd_messages setting
- Gate non-macOS overlay visibility on content-ready so first subtitle line is immediately hoverable and clickable
- Queue startup notifications in main process until overlay window finishes loading; upsert progress cards by id to avoid cold-start floods
- Defer background warmups until after overlay runtime init so queued notifications can deliver promptly
- Preserve character dictionary checking/building/importing/ready phases as distinct history entries; route building and importing to system notifications when notificationType is both

src/core/services/overlay-window.ts

@@ -116,6 +116,7 @@ export function createOverlayWindow(
     linuxX11FullscreenOverlay?: boolean;
     onVisibleWindowBlurred?: () => void;
     onVisibleWindowFocused?: () => void;
+    onWindowDidFinishLoad?: () => void;
     onWindowContentReady?: () => void;
     onWindowClosed: (kind: OverlayWindowKind, window: BrowserWindow) => void;
     yomitanSession?: Session | null;
@@ -139,6 +140,7 @@ export function createOverlayWindow(
   window.webContents.on('did-finish-load', () => {
     window.setTitle(OVERLAY_WINDOW_TITLES[kind]);
     options.onRuntimeOptionsChanged();
+    options.onWindowDidFinishLoad?.();
   });
 
   window.webContents.on('page-title-updated', (event) => {