refactor(auth): E.46 — extract logic-bearing factories (#611)

Three factories for the actions that carry real logic (not just
`set({ field: v })` pass-through):

- `createSkipStarActions` — skip-to-1★ counter with threshold check
  and per-`<activeServerId><trackId>` storage key. Disabling the
  feature wipes the counter map so a re-enable doesn't resume from
  stale partial counts. Crossing the threshold deletes the key so
  the next session starts fresh.
- `createMusicLibraryActions` — `setMusicFolders` falls back to
  `'all'` when the persisted filter points at a folder the server
  no longer reports; `setMusicLibraryFilter` bumps
  `musicLibraryFilterVersion` so subscribed pages refetch.
- `createPerServerCapabilityActions` — `setEntityRatingSupport`,
  `setAudiomuseNavidromeEnabled`, `setSubsonicServerIdentity`,
  `setInstantMixProbe`, `setAudiomuseNavidromeIssue`. Each branch
  knows which neighbouring per-server maps to wipe when the input
  invalidates them (identity not AudioMuse-eligible / probe empty /
  audiomuse disabled).

Pure code-move. authStore.ts: 384 → 270 LOC (−114). All trivial-setter
and logic-bearing action bodies are now out of the store body; only
the persist config + `onRehydrateStorage` migration block remains as
a non-factory chunk (target for E.47).

src/store/authMusicLibraryActions.ts

@@ -0,0 +1,47 @@
+import type { AuthState } from './authStoreTypes';
+
+type SetState = (
+  partial: Partial<AuthState> | ((state: AuthState) => Partial<AuthState>),
+) => void;
+type GetState = () => AuthState;
+
+/**
+ * Per-server music-folder selection. `setMusicFolders` is called
+ * after login / server change with the fresh Subsonic folder list;
+ * if the currently-persisted filter for that server points at a
+ * folder that no longer exists on the server, it falls back to
+ * `'all'` so the page doesn't end up filtering by a stale id.
+ *
+ * `setMusicLibraryFilter` writes the new filter and bumps
+ * `musicLibraryFilterVersion` so subscribed pages refetch their
+ * catalog data.
+ */
+export function createMusicLibraryActions(set: SetState, get: GetState): Pick<
+  AuthState,
+  'setMusicFolders' | 'setMusicLibraryFilter'
+> {
+  return {
+    setMusicFolders: (folders) => {
+      const sid = get().activeServerId;
+      set(s => {
+        const f = sid ? s.musicLibraryFilterByServer[sid] : undefined;
+        const invalidFilter = f && f !== 'all' && !folders.some(x => x.id === f);
+        return {
+          musicFolders: folders,
+          ...(sid && invalidFilter
+            ? { musicLibraryFilterByServer: { ...s.musicLibraryFilterByServer, [sid]: 'all' } }
+            : {}),
+        };
+      });
+    },
+
+    setMusicLibraryFilter: (folderId) => {
+      const sid = get().activeServerId;
+      if (!sid) return;
+      set(s => ({
+        musicLibraryFilterByServer: { ...s.musicLibraryFilterByServer, [sid]: folderId },
+        musicLibraryFilterVersion: s.musicLibraryFilterVersion + 1,
+      }));
+    },
+  };
+}