fix(subs): stop the lock/reload on source actions + regroup the row buttons

Lock/reload: every inline source action (check / backfill / recover / toggle /
remove) ended by refetching the WHOLE subscription list (store.loadAll /
refresh), which blocked the UI and re-rendered the table — collapsing the
expanded row, which read as "locks then resets." The action APIs already return
the updated source, so the store now patches that one row in place
(_patchSource / _dropSource); the post-action loadAll/refresh calls are gone.
Toggling enabled, starting/stopping a backfill, recovering, and removing are now
instant and leave the expansion intact.

Button regroup (operator-flagged: tiny, mis-clickable, not grouped by function):
- New shared SourceActions.vue used by desktop SourceRow + mobile SourceCard.
- Frequent actions stay as size="small" buttons: Check, Backfill/Stop.
- Low-frequency / destructive actions move into a labelled overflow (⋮) menu —
  Preview backfill, Recover dropped near-duplicates, Remove source — so they
  can't be fat-fingered, and the labels spell out recover-vs-backfill.
- Edit moves next to the source URL (its identity), out of the action cluster
  where it sat beside Remove.
- Single-source Remove now confirms (it had no guard before).

Tests: store patches/drops in place without dropping the cache.

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

frontend/test/sources.spec.js

@@ -41,6 +41,41 @@ describe('sources store', () => {
     expect(calls[0]).toContain('artist_id=7')
   })
 
+  it('update patches the source in place without dropping the cache', async () => {
+    const s = useSourcesStore()
+    s.byArtist.set(null, [
+      { id: 5, artist_id: 5, enabled: true, backfill_state: null },
+      { id: 6, artist_id: 5, enabled: true },
+    ])
+    stubFetch(() => ({
+      status: 200,
+      body: { id: 5, artist_id: 5, enabled: false, backfill_state: null },
+    }))
+    await s.update(5, { enabled: false }, 5)
+    // Cache survives (no full reload) and only source 5 changed.
+    expect(s.byArtist.has(null)).toBe(true)
+    const arr = s.byArtist.get(null)
+    expect(arr.find(r => r.id === 5).enabled).toBe(false)
+    expect(arr.find(r => r.id === 6).enabled).toBe(true)
+  })
+
+  it('startBackfill patches backfill_state in place', async () => {
+    const s = useSourcesStore()
+    s.byArtist.set(null, [{ id: 5, artist_id: 5, backfill_state: null }])
+    stubFetch(() => ({ status: 200, body: { id: 5, backfill_state: 'running' } }))
+    await s.startBackfill(5, 5)
+    expect(s.byArtist.get(null).find(r => r.id === 5).backfill_state).toBe('running')
+  })
+
+  it('remove drops the source from the cache in place', async () => {
+    const s = useSourcesStore()
+    s.byArtist.set(null, [{ id: 5, artist_id: 5 }, { id: 6, artist_id: 5 }])
+    stubFetch(() => ({ status: 204, body: null }))
+    await s.remove(5, 5)
+    expect(s.byArtist.has(null)).toBe(true)
+    expect(s.byArtist.get(null).map(r => r.id)).toEqual([6])
+  })
+
   it('create invalidates the all-cache and the artist-cache', async () => {
     const s = useSourcesStore()
     s.byArtist.set(null, [])