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

plugin/subminer/state.lua

@@ -35,6 +35,9 @@ function M.new()
 		auto_play_ready_osd_timer = nil,
 		auto_play_ready_signal_seen = false,
 		auto_play_ready_initial_pause_ownership_consumed = false,
+		overlay_loading_osd_active = false,
+		overlay_loading_osd_timer = nil,
+		overlay_loading_osd_frame = 1,
 		pending_visible_overlay_hide_timer = nil,
 		pending_visible_overlay_hide_generation = 0,
 		suppress_ready_overlay_restore = false,