feat: show mining frame toast on OSD and speed up stats session deletes

- Flash mined-frame screenshot as in-overlay image toast for OSD card notifications
- Batch cover art from stored DB blobs via POST /api/stats/covers (no extra AniList fetches)
- Show delete progress toast on stats home and sessions pages
- Refresh only affected rollups on session delete instead of full rebuild
- Rebuild lifetime summaries with aggregate SQL CTEs instead of per-session loop

src/shared/ipc/contracts.ts

@@ -144,6 +144,7 @@ export const IPC_CHANNELS = {
     subtitleSidebarToggle: 'subtitle-sidebar:toggle',
     primarySubtitleBarToggle: 'primary-subtitle-bar:toggle',
     configHotReload: 'config:hot-reload',
+    miningImage: 'mining:image',
   },
 } as const;