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>

static/v3/songs.js

@@ -949,7 +949,7 @@
             // address the local DB / filesystem). Both openers (⋮ and
             // right-click) share this list, so parity is structural.
             ...(state.provider === 'local' && song.filename ? [
-                { id: '__fixmatch', label: 'Fix match…' },
+                { id: '__fixmatch', label: 'Fix metadata…' },
                 { id: '__cover', label: 'Change cover…' },
                 { id: '__refreshmeta', label: 'Refresh metadata' },
                 { id: '__getinfo', label: 'Get info…' },