feat: add video quality selection for yt-dlp captures (#17)

- ytdlp::download() accepts quality: Option<&str>; quality_format()
  maps best/1080p/720p/480p/360p to yt-dlp -f format strings
- perform_capture() threads quality through to the downloader
- CaptureBody gains optional quality field; capture_handler validates
  it against the allowlist (400 on unknown values) before spawning
- CLI passes None (preserves existing best-quality behaviour)
- Frontend: isVideoSource() mirrors determine_source() exactly —
  shows quality picker only for yt-dlp-backed sources, excludes
  playlist/channel shorthands and tweet/thread paths
- submitCapture(archiveId, locator, quality) sends quality in POST body
- CSS: .capture-quality styles the inline select to fit the capture row
- Tests: quality_format unit tests in ytdlp.rs; two new route tests
  (valid quality accepted, invalid quality rejected with 400)
- Docs: video quality section added under Supported Platforms

frontend/src/styles.css

@@ -676,6 +676,39 @@ select {
 /* No bottom-margin on inputs inside rows; gap handles spacing */
 .capture-row-main .capture-input { margin-bottom: 0; }
 
+/* Quality selector shown for yt-dlp sources */
+.capture-quality {
+  flex-shrink: 0;
+  height: 32px;
+  padding: 0 6px;
+  border: 1px solid var(--line);
+  border-radius: var(--r);
+  background: var(--paper);
+  color: var(--ink);
+  font-size: 12px;
+  cursor: pointer;
+  outline: none;
+  transition: border-color .15s;
+}
+.capture-quality:hover { border-color: var(--accent-2); }
+.capture-quality:focus { border-color: var(--accent); }
+.capture-quality:disabled { opacity: 0.5; cursor: default; }
+/* Probing state: ellipsis shown while yt-dlp metadata is in flight */
+.capture-quality-probing {
+  flex-shrink: 0;
+  font-size: 12px;
+  color: var(--muted);
+  padding: 0 4px;
+  letter-spacing: .05em;
+}
+/* "No video detected" note */
+.capture-quality-hint {
+  flex-shrink: 0;
+  font-size: 12px;
+  color: var(--muted);
+  font-style: italic;
+}
+
 /* Status dot */
 .cap-dot {
   flex-shrink: 0;