fix(ui): full-width spectrum + log-mapped bars; deeper sepia + soft art fade

Spectrum
- Logarithmic frequency-to-bar mapping (squared time so bins
  stretch toward the highs). Per-bar high-end gain ramps from
  1.0x at the lowest bar to 3.0x at the highest, so the right
  half of the spectrum no longer reads as dead air.
- Floor bumped from 6% to 12% so silent bars stay visible.
- Skip bin 0 (DC + sub-rumble) which was overwhelming the lows.
- Use peak (not average) within each band — punchier visual.
- Container height 56→64, gradient now copper-lo → copper →
  copper-hi for more visible top tips. min-width: 0 / box-sizing
  border-box ensures the row truly claims the full grid column.
- Backend FFT path is unchanged: WS audio_data → setFrequencyData
  → renderVisualizerFrame → updateEditorialSpectrum. No
  client-side analyzer added.

Album art (vinyl label)
- Deeper sepia (0.35→0.6) and lower saturate (0.85→0.7) so
  vibrant covers blend into the copper grooves.
- Soft radial mask: outer ~22% of the disc fades toward the
  vinyl black so the album art dissolves into the surface
  rather than terminating at a hard clip edge.
- Hover state pulls the fade inward and eases sepia back so
  the user can still see the real cover at near-natural color.
- Glow tint matches the new sepia depth.

media_server/static/js/player.js

@@ -433,6 +433,10 @@ function renderVisualizerFrame() {
 }
 
 // ─── Editorial spectrum (.spectrum bars) driven by audio ──────
+// The bin distribution from the FFT is heavy on lows (the bass + mids
+// dominate); a linear mapping leaves the right half of the spectrum
+// looking dead. Use a logarithmic frequency-to-bar mapping plus a
+// per-bar high-end gain so all bars carry visible motion.
 function updateEditorialSpectrum(bins, numBins) {
     const root = document.querySelector('.now-playing .spectrum');
     if (!root) return;
@@ -440,15 +444,26 @@ function updateEditorialSpectrum(bins, numBins) {
     const barCount = bars.length;
     if (!barCount) return;
     document.body.classList.add('audio-spectrum-live');
+
+    // Skip the very lowest bin (DC + sub-rumble) which often dominates.
+    const lowBin = 1;
+    const highBin = numBins - 1;
     for (let i = 0; i < barCount; i++) {
-        // Map each visual bar to a chunk of frequency bins (averaged).
-        const startIdx = Math.floor((i / barCount) * numBins);
-        const endIdx = Math.max(startIdx + 1, Math.floor(((i + 1) / barCount) * numBins));
-        let sum = 0;
-        for (let j = startIdx; j < endIdx && j < numBins; j++) sum += bins[j];
-        const avg = sum / (endIdx - startIdx);
-        // Boost mids/highs and floor to 6% so quiet bars are still visible.
-        const pct = Math.max(6, Math.min(100, avg * 110));
+        // Logarithmic mapping: equal-area slices of the audible spectrum
+        // map to equal numbers of bars. Each bar covers a wider bin range
+        // toward the highs so they get amplified naturally.
+        const t0 = i / barCount;
+        const t1 = (i + 1) / barCount;
+        const startIdx = Math.max(lowBin, Math.floor(lowBin + Math.pow(t0, 2.0) * (highBin - lowBin)));
+        const endIdx = Math.max(startIdx + 1, Math.floor(lowBin + Math.pow(t1, 2.0) * (highBin - lowBin)));
+        let peak = 0;
+        for (let j = startIdx; j < endIdx && j < numBins; j++) {
+            if (bins[j] > peak) peak = bins[j];
+        }
+        // Per-bar high-end gain: 1.0 at the lowest bar, ~3.0 at the highest.
+        const gain = 1 + (i / barCount) * 2.0;
+        // Floor at 12% so silent bars are still visually present.
+        const pct = Math.max(12, Math.min(100, peak * 110 * gain));
         bars[i].style.height = pct + '%';
     }
 }