fix(player): real audio level on VU; full-width spectrum; hide canvas under vinyl

VU needle reflects actual audio output
- Was just a synthetic wobble bounded by the volume slider value.
  Now reads RMS of the FFT bins (skipping bin 0 / DC) the visualizer
  feeds in, multiplies by current volume, and applies attack/release
  smoothing for analog-feeling ballistics.
- Falls back to the synthetic wobble when audio capture isn't
  running so the needle still looks alive on the static fallback.
- When playback stops, needle settles to the bottom of the swing
  (-45deg) instead of holding the volume position.

Spectrum width — actually fixed this time
- Root cause: CSS repeat() does NOT accept a CSS variable for its
  count argument, so my `repeat(var(--spectrum-bars), 1fr)` rule
  was invalid and silently dropped, leaving the legacy/auto sizing
  behavior. Set grid-template-columns directly from JS to
  `repeat(40, minmax(0, 1fr))`.
- CSS retains a `repeat(40, minmax(0, 1fr))` literal as a default
  so the row renders sane even before JS executes.

Spectrogram canvas under vinyl
- Hidden via display: none — the editorial .spectrum row already
  shows the audio spectrum; the canvas was redundant and ugly.
  Element stays in DOM so the visualizer JS keeps rendering (drives
  album-art bass-pulse + dynamic background bands).

media_server/static/js/app.js

@@ -172,9 +172,11 @@ window.addEventListener('DOMContentLoaded', async () => {
     const spectrumRoot = document.getElementById('player-spectrum');
     if (spectrumRoot && !spectrumRoot.children.length) {
         const SPECTRUM_BARS = 40;
-        // Sync the grid column count to the bar count so the row truly
-        // fills the column even if the bar count changes later.
-        spectrumRoot.style.setProperty('--spectrum-bars', SPECTRUM_BARS);
+        // CSS repeat() doesn't accept a var() for its count — set the
+        // grid column template from JS so it always matches the bar
+        // count and stretches each bar to claim 1fr of the row.
+        spectrumRoot.style.gridTemplateColumns =
+            `repeat(${SPECTRUM_BARS}, minmax(0, 1fr))`;
         const frag = document.createDocumentFragment();
         for (let i = 0; i < SPECTRUM_BARS; i++) {
             const s = document.createElement('span');