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/renderer/style.css

@@ -146,6 +146,37 @@ body:focus-visible,
   transform: translateY(0);
 }
 
+.mining-image-toast {
+  position: absolute;
+  top: 16px;
+  right: 16px;
+  padding: 6px;
+  border-radius: 10px;
+  border: 1px solid rgba(138, 213, 202, 0.45);
+  background: linear-gradient(135deg, rgba(10, 44, 40, 0.94), rgba(8, 28, 33, 0.94));
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.45);
+  pointer-events: none;
+  opacity: 0;
+  transform: translateY(-6px);
+  transition:
+    opacity 160ms ease,
+    transform 160ms ease;
+  z-index: 1300;
+}
+
+.mining-image-toast-image {
+  display: block;
+  width: 128px;
+  height: 128px;
+  border-radius: 6px;
+  object-fit: cover;
+}
+
+.mining-image-toast:not(.hidden) {
+  opacity: 1;
+  transform: translateY(0);
+}
+
 .modal {
   position: absolute;
   inset: 0;