perf(explore): ask the disk once, prefetch once, and menu the releases

Three things on the Explore surfaces, all about not asking twice.

A portrait already on disk costs no network call. explore-view seeded
only from the library store — owned artists, which on a catalog search
is nearly none of the results — and sent everything else to
GetArtistImageURL, the resolving entry point, one await at a time.
GetArtistImagesCachedPaths asks the disk about every unresolved artist
in one call, and only what it does not answer reaches the resolver,
in parallel.

The artist page's two sections both wanted PrefetchReleases and each
called it, so the most expensive call the app makes was issued twice
for an overlapping set on a 1 req/s limiter. They are collected and
sent once on a microtask, and prefetchRequested stops the cold-artist
refetch re-asking for what it already asked for.

The release cards — most of the artist page — had no context menu at
all. They have one now on both release shapes, normalised to a
ReleaseMenuTarget when the menu opens so the union does not reach the
action handlers. It is a discriminated union rather than one nullable
field per kind because the panel is shared with the track menu: that is
what keeps aria-label moving with the target, which is the fault
cover-grid shipped. Which items appear is three different questions —
playback is gated on a local album id, not on "owned", and the request
needs a catalog MBID, so it is absent for a library-only release.

Note on the docs: the CLAUDE.md and NOTES.md prose here was
reconstructed after a mishandled `git stash --keep-index` destroyed the
uncommitted originals. One NOTES.md section is marked as incomplete
where its text could not be recovered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UDCbcCZQepnpSQYJ6SxxZm

frontend/src/utils/explore-link.ts

@@ -80,7 +80,7 @@ async function findLocalAlbum(
 /** Find the library artist row for a name, loading the cache if needed. */
 async function findLocalArtist(
     artistName: string,
-): Promise<{ ID: number; Name: string } | null> {
+): Promise<{ ID: number; Name: string; MBID: string } | null> {
     let artists = libraryStore.cachedArtists;
 
     if (!artists) {
@@ -176,9 +176,12 @@ export function artistLink(
             const local = await findLocalArtist(artistName);
             if (!local) return;
 
+            // The caller's row had no MBID, but the library row for the
+            // same artist may — the grid routes by exactly this field,
+            // so reading it here is what keeps the two paths agreeing.
             navigate(target, {
                 view: 'explore-artist-details',
-                artistMBID: '',
+                artistMBID: local.MBID || '',
                 artistName,
                 localArtistId: local.ID,
             });