fix(visualizer): full-width spectrum + device pick auto-starts capture

Spectrum width
- grid-auto-flow: column with implicit columns wasn't reliably
  stretching to fill the parent. Switch to explicit
  grid-template-columns: repeat(var(--spectrum-bars), minmax(0, 1fr))
  with the bar count exposed as a CSS variable from JS so the
  column count and the actual bar count stay in sync.
- !important on display/grid-template-columns/width to defeat any
  legacy descendant rules.

Device selection
- Picking a device in the audio-device dropdown is an explicit
  signal that the user wants capture. Auto-enable the visualizer
  if it isn't already on, then call applyVisualizerMode so the
  WS subscription happens and the badge flips from 'Available' to
  'Active'. Was only doing this when visualizer was already on,
  which is why the user kept seeing 'Available, not capturing'.

media_server/static/js/player.js

@@ -618,7 +618,12 @@ export async function onAudioDeviceChanged() {
             const result = await resp.json();
             updateAudioDeviceStatus({ available: result.success, ...result });
             await checkVisualizerAvailability();
-            if (visualizerEnabled) applyVisualizerMode();
+            // Picking a device is an explicit signal the user wants
+            // capture: auto-enable the visualizer if it isn't already on.
+            if (!visualizerEnabled && visualizerAvailable) {
+                setVisualizerEnabled(true);
+            }
+            applyVisualizerMode();
             showToast(t('settings.audio.device_changed'), 'success');
         } else {
             showToast(t('settings.audio.device_change_failed'), 'error');