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

docs-site/character-dictionary.md

@@ -158,6 +158,8 @@ The three collapsible sections can be configured to start open or closed:
 
 When `subtitleStyle.nameMatchEnabled` is `true`, SubMiner runs an auto-sync routine whenever the active media changes.
 
+These phases are emitted through the configured notification surface. Some phases are skipped when unnecessary: `generating` only appears on a cache miss, `building` only appears when the merged ZIP must be rebuilt, and `importing` only appears when Yomitan needs a new dictionary import.
+
 **Phases:**
 
 1. **checking** - Is there already a cached snapshot for this media ID?