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

internal/web/router.go

@@ -69,6 +69,7 @@ func (h *Web) NewRouter() http.Handler {
 	mux.HandleFunc("POST /settings/sessions/{id}/revoke", h.requireUser(h.handleSessionRevoke))
 
 	mux.HandleFunc("GET /stream/{id}", h.requireUser(h.handleStream))
+	mux.HandleFunc("GET /player/lyrics/{id}", h.requireUser(h.handleLyrics))
 	mux.HandleFunc("GET /cover-art/{id}", h.requireUser(h.handleCoverArt))
 
 	mux.HandleFunc("GET /admin", h.requireAdmin(h.handleAdminRoot))