Add a web player lyrics panel with active-line highlighting and click-to-seek

New GET /player/lyrics/{id} (auth-required) returns the best available
stored structuredLyrics entry for a song — preferring a synced one —
as plain JSON lines with a start time only where the source actually
has one.

The player bar gains a lyrics toggle; the panel fetches lyrics on
every song change, renders each line, and — only for lines that
actually carry a startMs — makes them clickable (seeks + plays) and
highlights the currently active line as playback progresses. Plain
(unsynced) lyrics render as static text with no fabricated timing or
highlighting.

Note: verified via Go template parsing/rendering and the new handler
tests, plus a JS syntax check; not manually exercised in a browser in
this session.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

web/static/css/app.css

@@ -286,6 +286,44 @@ body {
   flex: 0 0 auto;
 }
 
+.vs-lyrics-panel {
+  position: fixed;
+  left: 0;
+  right: 0;
+  bottom: var(--vs-player-height);
+  max-height: 40vh;
+  overflow-y: auto;
+  background: var(--bs-body-bg);
+  border-top: 1px solid var(--bs-border-color);
+  padding: 1rem;
+  z-index: 1029;
+  text-align: center;
+}
+
+.vs-lyrics-line {
+  padding: 0.15rem 0;
+  color: var(--bs-secondary-color);
+  transition: color 0.15s ease;
+}
+
+.vs-lyrics-line.vs-lyrics-synced {
+  cursor: pointer;
+}
+
+.vs-lyrics-line.vs-lyrics-synced:hover {
+  color: var(--bs-body-color);
+}
+
+.vs-lyrics-line.vs-lyrics-active {
+  color: var(--bs-body-color);
+  font-weight: 600;
+}
+
+.vs-lyrics-empty {
+  color: var(--bs-secondary-color);
+  font-size: 0.85rem;
+}
+
 /* --- Misc --- */
 
 .vs-star-btn {