feat(server,frontend): playlist quality selector + per-video overrides + sync UI

routes.rs:
- CaptureBody gains per_item_quality (HashMap<String,String>, serde(default))
  and sync (bool, serde(default)); both validated before use
  - per_item_quality values validated against same quality predicate as top-level
    quality field ("best"|"audio"|"NNNp") so bad per-video values are rejected
    at the API boundary rather than silently falling through to quality_format
- capture_handler threads body.per_item_quality + body.sync into CaptureConfig
  (replaces hardcoded empty stubs); rearchive_handler keeps empty defaults
- New POST /api/archives/:id/captures/probe-playlist: calls
  probe_playlist_qualities via spawn_blocking; 400 for non-playlist locator,
  502 on yt-dlp failure, returns PlaylistProbeResult as JSON

api.js:
- probePlaylist(archiveId, locator): POST probe-playlist endpoint
- submitCapture: forwards per_item_quality (non-empty) and sync:true from
  extraExtensions param added to submitBgJob

CaptureDialog.jsx:
- isPlaylistSource(): detects yt:/youtube: playlist/@/channel, ytm:playlist/,
  YouTube/YTM HTTP(S) URLs with list= param or channel pathnames
- makeItem(): 6 new playlist state fields
- applyPlaylistQuality(): conflict logic — videos that can reach selected
  quality get it set; videos that can't and have no prior selection are left
  null (conflict); videos with a prior selection keep it when quality is raised
- hasConflict(): any playlistItems entry with quality===null
- updateLocator(): isPlaylistSource branch with 800ms debounce→probePlaylist;
  existing isVideoSource path unchanged
- Archive button disabled when anyConflict or any probe in flight
- Per-video expand list with individual quality selects, conflict badges,
  sync toggle (appears after probe completes)

styles.css: playlist expansion, conflict, sync toggle CSS

frontend/src/styles.css

@@ -2866,3 +2866,77 @@ body.has-audio-bar { padding-bottom: 56px; }
   font-size: 0.85em;
   opacity: 0.55;
 }
+
+/* ── Playlist quality expansion ──────────────────────────── */
+.capture-playlist-toggle {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 0.75rem;
+  color: var(--muted);
+  background: none;
+  border: none;
+  cursor: pointer;
+  padding: 2px 6px;
+}
+.capture-playlist-toggle:hover { color: var(--text); }
+
+.capture-conflict-badge {
+  font-size: 0.7rem;
+  color: #c07000;
+  background: #fff3cd;
+  border: 1px solid #e0a000;
+  border-radius: 4px;
+  padding: 1px 6px;
+  margin-left: 6px;
+}
+
+.capture-playlist-items {
+  border-top: 1px solid var(--line-soft);
+  padding: 4px 0;
+  max-height: 300px;
+  overflow-y: auto;
+}
+
+.capture-playlist-item {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 4px 12px;
+  font-size: 0.8rem;
+}
+.capture-playlist-item--conflict {
+  background: #fff8f0;
+}
+.capture-playlist-item-title {
+  flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  color: var(--text);
+}
+.capture-item-quality {
+  font-size: 0.75rem;
+  padding: 2px 4px;
+  border: 1px solid var(--line);
+  border-radius: 4px;
+  background: var(--bg);
+  color: var(--text);
+}
+.capture-playlist-conflict-badge {
+  font-size: 0.7rem;
+  color: #c07000;
+  white-space: nowrap;
+}
+
+.capture-sync-row {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 12px;
+  font-size: 0.78rem;
+  color: var(--muted);
+  border-top: 1px solid var(--line-soft);
+  cursor: pointer;
+}
+.capture-sync-row input[type=checkbox] { cursor: pointer; }