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/authPerServerCapabilityActions.ts

@@ -0,0 +1,93 @@
+import { isNavidromeAudiomuseSoftwareEligible } from '../utils/subsonicServerIdentity';
+import type { AuthState } from './authStoreTypes';
+
+type SetState = (
+  partial: Partial<AuthState> | ((state: AuthState) => Partial<AuthState>),
+) => void;
+
+/**
+ * Per-server capability flags learned from pings/probes:
+ *
+ * - `setEntityRatingSupport` — does setRating apply to album/artist
+ *   ids on this server? Stored as `unknown`/`yes`/`no`.
+ * - `setAudiomuseNavidromeEnabled` — user opted in/out of the
+ *   AudioMuse-AI Instant Mix path for this Navidrome. Disable also
+ *   clears the related issue flag.
+ * - `setSubsonicServerIdentity` — server's identity from ping. If the
+ *   ping reveals the server isn't AudioMuse-eligible (wrong type or
+ *   too old), wipe the related caps for that id so the UI doesn't
+ *   keep a stale toggle.
+ * - `setInstantMixProbe` — probe result for getSimilarSongs. If
+ *   `empty`, wipe the related AudioMuse caps so the row hides.
+ * - `setAudiomuseNavidromeIssue` — set/clear the "current session
+ *   saw a failure" flag.
+ */
+export function createPerServerCapabilityActions(set: SetState): Pick<
+  AuthState,
+  | 'setEntityRatingSupport'
+  | 'setAudiomuseNavidromeEnabled'
+  | 'setSubsonicServerIdentity'
+  | 'setInstantMixProbe'
+  | 'setAudiomuseNavidromeIssue'
+> {
+  return {
+    setEntityRatingSupport: (serverId, level) =>
+      set(s => ({
+        entityRatingSupportByServer: { ...s.entityRatingSupportByServer, [serverId]: level },
+      })),
+
+    setAudiomuseNavidromeEnabled: (serverId, enabled) =>
+      set(s => {
+        const audiomuseNavidromeByServer = enabled
+          ? { ...s.audiomuseNavidromeByServer, [serverId]: true }
+          : (() => {
+              const { [serverId]: _removed, ...rest } = s.audiomuseNavidromeByServer;
+              return rest;
+            })();
+        const { [serverId]: _issueRm, ...issueRest } = s.audiomuseNavidromeIssueByServer;
+        return { audiomuseNavidromeByServer, audiomuseNavidromeIssueByServer: issueRest };
+      }),
+
+    setSubsonicServerIdentity: (serverId, identity) =>
+      set(s => {
+        const subsonicServerIdentityByServer = { ...s.subsonicServerIdentityByServer, [serverId]: { ...identity } };
+        if (!isNavidromeAudiomuseSoftwareEligible(identity)) {
+          const { [serverId]: _a, ...audiomuseRest } = s.audiomuseNavidromeByServer;
+          const { [serverId]: _i, ...issueRest } = s.audiomuseNavidromeIssueByServer;
+          const { [serverId]: _p, ...probeRest } = s.instantMixProbeByServer;
+          return {
+            subsonicServerIdentityByServer,
+            audiomuseNavidromeByServer: audiomuseRest,
+            audiomuseNavidromeIssueByServer: issueRest,
+            instantMixProbeByServer: probeRest,
+          };
+        }
+        return { subsonicServerIdentityByServer };
+      }),
+
+    setInstantMixProbe: (serverId, result) =>
+      set(s => {
+        const instantMixProbeByServer = { ...s.instantMixProbeByServer, [serverId]: result };
+        if (result === 'empty') {
+          const { [serverId]: _a, ...audiomuseRest } = s.audiomuseNavidromeByServer;
+          const { [serverId]: _i, ...issueRest } = s.audiomuseNavidromeIssueByServer;
+          return {
+            instantMixProbeByServer,
+            audiomuseNavidromeByServer: audiomuseRest,
+            audiomuseNavidromeIssueByServer: issueRest,
+          };
+        }
+        return { instantMixProbeByServer };
+      }),
+
+    setAudiomuseNavidromeIssue: (serverId, hasIssue) =>
+      set(s =>
+        hasIssue
+          ? { audiomuseNavidromeIssueByServer: { ...s.audiomuseNavidromeIssueByServer, [serverId]: true } }
+          : (() => {
+              const { [serverId]: _rm, ...rest } = s.audiomuseNavidromeIssueByServer;
+              return { audiomuseNavidromeIssueByServer: rest };
+            })(),
+      ),
+  };
+}