feat(library): 3-tab Fix-metadata popup — Details / Cover art / Match (slice 4)

Turns the thin single-song fix-match modal into the Plex-style metadata
editor reached from a card's "Fix metadata…" menu:

- Details tab: type + lock the displayed title/artist/album/year. Values
  ride the reversible override store (GET/PUT /api/song/{fn}/overrides); each
  field sits on its pack value (Yours/Pack provenance + revert-to-pack), a lock
  pins it against auto-match, and Save repaints the grid via library:changed
  (slice-3 overlay). This is the real tool for the blank-artist city-pop pile
  MusicBrainz can't surface — you just type the right title.
- Cover art tab: hands off to the shared image picker (its own modal); the
  pick refreshes the thumbnail everywhere.
- Match tab: the existing MusicBrainz search + candidate/pick flow, refactored
  into shared body/footer helpers (the queue-review flow is untouched).

Backend: GET /overrides now also returns the pack baseline so the Details tab
can pre-fill + show provenance. tailwind.min.css regenerated (build-tailwind.sh)
for the popup's new utility classes.

Identify-by-audio (AcoustID) is deferred: it lives in unmerged PR #759, off
main — the Match tab gains the button once #759 lands and this branch rebases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_field_overrides.py

@@ -117,6 +117,19 @@ def test_api_put_get_and_clear(client, server):
     assert "artist" not in client.get("/api/song/a.archive/overrides").json()["overrides"]
 
 
+def test_api_get_returns_pack_values(client, server):
+    _put(server, "a.archive", title="Pack Title", artist="Pack Artist",
+         album="Pack Album", year="1988")
+    server.meta_db.set_song_override("a.archive", "title", value="Fixed Title")
+    body = client.get("/api/song/a.archive/overrides").json()
+    # the override rides "overrides"; the pack baseline rides "pack" (all 5 fields)
+    assert body["overrides"]["title"]["value"] == "Fixed Title"
+    assert body["pack"] == {"title": "Pack Title", "artist": "Pack Artist",
+                            "album": "Pack Album", "year": "1988", "genre": ""}
+    # a song with no row still gets an all-empty pack (popup always has values)
+    assert client.get("/api/song/ghost.archive/overrides").json()["pack"]["title"] == ""
+
+
 def test_api_rejects_unknown_field(client, server):
     _put(server, "a.archive")
     r = client.put("/api/song/a.archive/overrides",