fix(visualizer): auto-enable actually starts capture; persist audio device

Auto-enable was a no-op
- Writing 'visualizerEnabled'='true' to localStorage from app.js did
  not update the exported `let visualizerEnabled` in player.js. So
  applyVisualizerMode() saw the stale `false` and went into the
  DISABLE branch — leaving the device 'available, not capturing'.
- Add a setVisualizerEnabled() setter exported from player.js and
  call it before applyVisualizerMode() during boot.

Audio device persistence
- Save the selected device name to localStorage on change.
- On loadAudioDevices(), prefer status.current_device (server's
  current state) but fall back to the localStorage value if the
  server doesn't know one (e.g. after a server restart).
- If the saved device wasn't recognized by the server, push it back
  via POST /api/media/visualizer/device so capture lands on it
  immediately. Best-effort; no toast on failure.

media_server/static/js/app.js

@@ -22,7 +22,7 @@ import {
     initTheme, toggleTheme, initAccentColor, applyAccentColor,
     renderAccentSwatches, selectAccentColor, toggleAccentPicker, lightenColor,
     toggleVinylMode, applyVinylMode,
-    visualizerEnabled, visualizerAvailable,
+    visualizerEnabled, visualizerAvailable, setVisualizerEnabled,
     checkVisualizerAvailability, toggleVisualizer, applyVisualizerMode,
     loadAudioDevices, onAudioDeviceChanged,
     setupProgressDrag, updateUI, updatePlaybackState, stopPositionInterpolation,
@@ -186,14 +186,13 @@ window.addEventListener('DOMContentLoaded', async () => {
     // Initialize audio visualizer — auto-enable when supported so the
     // spectrum shows real audio out of the box.
     checkVisualizerAvailability().then(() => {
-        if (visualizerAvailable && !visualizerEnabled) {
-            // Auto-enable on first install if loopback capture works.
-            if (localStorage.getItem('visualizerEnabled') === null) {
-                localStorage.setItem('visualizerEnabled', 'true');
-            }
-        }
-        if ((visualizerEnabled || localStorage.getItem('visualizerEnabled') === 'true')
-            && visualizerAvailable) {
+        if (!visualizerAvailable) return;
+        // First install: opt the user in by default since the spectrum
+        // is the centerpiece of the player view.
+        const stored = localStorage.getItem('visualizerEnabled');
+        const shouldEnable = stored === null ? true : stored === 'true';
+        if (shouldEnable) {
+            setVisualizerEnabled(true);   // updates the let in player.js
             applyVisualizerMode();
         }
     });