refactor(decouple): playback-engine bridge — break authStore→engine edges

The last core→engine inversions blocking the playback-core move live in the
authStore settings/profile family: authServerProfileActions reads
usePlayerStore.queueServerId + calls clearQueueServerForPlayback (on server
delete), and authAudioSettingsActions calls
usePlayerStore.updateReplayGainForCurrentTrack from ~8 ReplayGain/normalization
setters. Both are re-exported through the authStore barrel, so a global store
would depend on the (soon-to-move) engine.

Add core seam store/playbackEngineBridge.ts: a registry exposing getQueueServerId
/ clearQueueServerForPlayback / updateReplayGainForCurrentTrack with no-op/null
defaults. The engine registers its impls via store/playbackEngineBridgeRegister.ts
(side-effect-imported by MainApp at boot). authStore actions now call the neutral
delegators instead of importing the engine.

Default is safe: the only callers are user-triggered settings/profile actions that
fire long after boot (engine already registered); at boot there's no current track
or queue binding, so the no-op would be correct anyway. authStore.servers.test
imports the register module so removeServer's queue-clear runs through the real
engine wiring.

Prepares the playback-core move (Engine → features/playback): with this + keeping
the 3 config-helper utils in utils/ + tolerating 2 type-only edges, the engine has
no inbound core→feature inversions left. tsc 0, lint 0, suite 319/2353 green.
Behavior-touching (server-delete queue clear, settings→gain refresh) → Frank QA.

src/app/MainApp.tsx

@@ -1,4 +1,5 @@
 import { initAudioListeners } from '../store/initAudioListeners';
+import '../store/playbackEngineBridgeRegister'; // installs the playback-engine bridge at boot
 import { lazy, Suspense, useEffect, useState } from 'react';
 import { BrowserRouter, Route, Routes } from 'react-router-dom';
 import { invoke } from '@tauri-apps/api/core';