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/authSkipStarActions.ts

@@ -0,0 +1,65 @@
+import {
+  clampSkipStarThreshold,
+  skipStarCountStorageKey,
+} from './authStoreHelpers';
+import type { AuthState } from './authStoreTypes';
+
+type SetState = (
+  partial: Partial<AuthState> | ((state: AuthState) => Partial<AuthState>),
+) => void;
+type GetState = () => AuthState;
+
+/**
+ * Skip-to-1★ feature: after N manual skips of the same track, set
+ * its rating to 1 if still unrated. Counter is per
+ * `<activeServerId><trackId>` key, persisted in
+ * `skipStarManualSkipCountsByKey`, and cleared either when the
+ * threshold is crossed (so the next session starts fresh) or when
+ * the track ends naturally without a skip.
+ *
+ * Disabling the feature wipes the counter map so re-enabling later
+ * doesn't resume from stale partial counts.
+ */
+export function createSkipStarActions(set: SetState, get: GetState): Pick<
+  AuthState,
+  | 'setSkipStarOnManualSkipsEnabled'
+  | 'setSkipStarManualSkipThreshold'
+  | 'recordSkipStarManualAdvance'
+  | 'clearSkipStarManualCountForTrack'
+> {
+  return {
+    setSkipStarOnManualSkipsEnabled: (v) =>
+      set({
+        skipStarOnManualSkipsEnabled: v,
+        ...(v ? {} : { skipStarManualSkipCountsByKey: {} }),
+      }),
+    setSkipStarManualSkipThreshold: (v) =>
+      set({ skipStarManualSkipThreshold: clampSkipStarThreshold(v) }),
+
+    recordSkipStarManualAdvance: (trackId: string) => {
+      const s = get();
+      if (!s.skipStarOnManualSkipsEnabled || s.skipStarManualSkipThreshold < 1) return null;
+      const key = skipStarCountStorageKey(s.activeServerId, trackId);
+      const prev = s.skipStarManualSkipCountsByKey[key] ?? 0;
+      const threshold = s.skipStarManualSkipThreshold;
+      const next = prev + 1;
+      if (next >= threshold) {
+        const { [key]: _removed, ...rest } = s.skipStarManualSkipCountsByKey;
+        set({ skipStarManualSkipCountsByKey: rest });
+        return { crossedThreshold: true };
+      }
+      set({
+        skipStarManualSkipCountsByKey: { ...s.skipStarManualSkipCountsByKey, [key]: next },
+      });
+      return { crossedThreshold: false };
+    },
+
+    clearSkipStarManualCountForTrack: (trackId: string) => {
+      const s = get();
+      const key = skipStarCountStorageKey(s.activeServerId, trackId);
+      if (s.skipStarManualSkipCountsByKey[key] === undefined) return;
+      const { [key]: _removed, ...rest } = s.skipStarManualSkipCountsByKey;
+      set({ skipStarManualSkipCountsByKey: rest });
+    },
+  };
+}