refactor(network): decouple network-reachability layer from features → lib/network

The connection-reachability + Subsonic network-guard helpers were pinned low by
four lib/api consumers (subsonicLibrary/Playlists/Ratings/Scrobble) yet ran two
lower-layer→feature inversions. Both removed without a registry:

- devOfflineBrowseStore is a self-contained DEV-only toggle with zero offline-
  feature coupling — it was only colocated there. Relocated features/offline/store
  → store/ (global). The @/features/offline barrel re-exports it so feature/UI
  consumers are unchanged; the three lower-layer readers (subsonicNetworkGuard,
  activeServerReachability, useConnectionStatus) now import it from @/store directly.

- subsonicNetworkGuard's only other feature dep was playback's resolvePlaybackUrl,
  used solely for the psysonic-local:// skip check. Added hasLocalPlaybackUrl to the
  existing M4 substrate store/localPlaybackResolve — it mirrors resolvePlaybackUrl's
  local-source branch exactly (same profile resolution; the empty-serverId playback
  fallback never applies in the guard), so the skip stays bit-identical.

network/ (subsonicNetworkGuard + activeServerReachability + tests) → lib/network,
now @/features-free. useConnectionStatus is now iron-rule-clean and stays in hooks/
(cross-cutting). Test mocks retargeted to the new seam modules.

Behavior-adjacent (covered by suite; default paths identical): the local-bytes skip
helper — flag for offline-playback runtime QA alongside the M4 media-resolver seam.

src/lib/network/activeServerReachability.test.ts

@@ -0,0 +1,46 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { useDevOfflineBrowseStore } from '@/store/devOfflineBrowseStore';
+import {
+  getActiveServerReachable,
+  isActiveServerReachable,
+  onActiveServerBecameReachable,
+  resetActiveServerConnectionSnapshot,
+  setActiveServerReachable,
+} from '@/lib/network/activeServerReachability';
+
+describe('activeServerReachability', () => {
+  beforeEach(() => {
+    useDevOfflineBrowseStore.setState({ forceOffline: false });
+    resetActiveServerConnectionSnapshot();
+  });
+
+  it('isActiveServerReachable requires an explicit successful probe', () => {
+    expect(isActiveServerReachable()).toBe(false);
+    setActiveServerReachable(true);
+    expect(isActiveServerReachable()).toBe(true);
+    setActiveServerReachable(false);
+    expect(isActiveServerReachable()).toBe(false);
+  });
+
+  it('exposes the last probe result', () => {
+    setActiveServerReachable(true);
+    expect(getActiveServerReachable()).toBe(true);
+  });
+
+  it('isActiveServerReachable is false when DEV force-offline is enabled', () => {
+    if (!import.meta.env.DEV) return;
+    setActiveServerReachable(true);
+    useDevOfflineBrowseStore.setState({ forceOffline: true });
+    expect(isActiveServerReachable()).toBe(false);
+  });
+
+  it('onActiveServerBecameReachable fires only on false/null → true', () => {
+    const listener = vi.fn();
+    onActiveServerBecameReachable(listener);
+    setActiveServerReachable(false);
+    setActiveServerReachable(true);
+    expect(listener).toHaveBeenCalledTimes(1);
+    setActiveServerReachable(true);
+    expect(listener).toHaveBeenCalledTimes(1);
+  });
+});