feat(0.4.0): now-playing indicator — dancing bars + Web Audio visualizer

Two-layer "this is playing right now" visual:

(1) CSS dancing bars — four tiny vertical bars next to the "Now Playing"
    label, staggered `@keyframes` pulse while audio plays. Pure CSS, no
    JS dependency, tints to the user's WP admin colour scheme via
    var(--wp-admin-theme-color). Driven by a single `.is-playing` class
    on `.radio-player` toggled from the existing play/pause/error
    handlers. Always works.

(2) Web Audio frequency visualizer (progressive upgrade) — on first play,
    builds AudioContext + AnalyserNode + canvas drawing pipeline. When
    the analyser starts returning real (non-zero) data, hides the bars
    and shows the canvas with live frequency bars. Falls back to bars
    if AudioContext is unavailable, createMediaElementSource throws, or
    the analyser returns all-zeros for >2s (CORS silently blocking).
    State machine on player._vizState: no-webaudio / init-failed /
    cors-blocked / ok.

`<audio>` element gained `crossorigin="anonymous"` so Web Audio can read
the stream data (SomaFM serves the CORS headers).

Files: radio.php (version), inc/admin-page.php + inc/dashboard-widget.php
(.radio-player__indicator with .radio-player__bars + canvas; crossorigin
on audio), assets/css/radio.css (indicator, bars, radio-bars-dance
keyframes, canvas size), assets/js/radio.js (tryVisualizer,
startVizLoop, stopVizLoop; play/pause/error handlers wire the loop and
toggle is-playing), inc/about.php (history entry).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

assets/css/radio.css

@@ -30,6 +30,73 @@
     margin-right: 4px;
 }
 
+/* ──────────────────────────────────────────────────────────────────
+ * Now-playing indicator — CSS dancing bars (always present) +
+ * optional Web Audio frequency visualizer (canvas, progressive
+ * upgrade). The JS toggles a `.is-playing` class on the parent
+ * `.radio-player` to drive the dance; when the visualizer is
+ * successfully wired, it hides the bars and reveals the canvas.
+ * ─────────────────────────────────────────────────────────────── */
+
+.radio-player__indicator {
+    display: inline-flex;
+    align-items: flex-end;
+    height: 14px;
+    min-width: 18px;
+    margin-right: 6px;
+    vertical-align: middle;
+    overflow: hidden;
+}
+
+.radio-player__bars {
+    display: inline-flex;
+    align-items: flex-end;
+    height: 100%;
+    width: 18px;
+    gap: 2px;
+}
+
+.radio-player__bars span {
+    display: inline-block;
+    width: 2px;
+    height: 100%;
+    background: var(--wp-admin-theme-color, #2271b1);
+    border-radius: 1px;
+    transform-origin: bottom;
+    transform: scaleY(0.25);
+    transition: transform 0.2s ease;
+    opacity: 0.65;
+}
+
+/* When playing, each bar dances with a staggered delay so the row
+   has a lifelike, slightly out-of-phase pulse rather than uniform
+   thumping. ~0.85s loop keeps it lively without being twitchy. */
+.radio-player.is-playing .radio-player__bars span {
+    animation: radio-bars-dance 0.85s ease-in-out infinite;
+    opacity: 1;
+}
+.radio-player.is-playing .radio-player__bars span:nth-child(1) { animation-delay: 0s;   }
+.radio-player.is-playing .radio-player__bars span:nth-child(2) { animation-delay: 0.18s; }
+.radio-player.is-playing .radio-player__bars span:nth-child(3) { animation-delay: 0.36s; }
+.radio-player.is-playing .radio-player__bars span:nth-child(4) { animation-delay: 0.09s; }
+
+@keyframes radio-bars-dance {
+    0%, 100% { transform: scaleY(0.3); }
+    20%      { transform: scaleY(0.9); }
+    40%      { transform: scaleY(0.5); }
+    60%      { transform: scaleY(1);   }
+    80%      { transform: scaleY(0.65); }
+}
+
+/* Web Audio canvas — wider than the CSS bars so the live frequency
+   data has room to breathe. JS swaps display when the visualizer
+   confirms it's receiving real (non-zero) data. */
+.radio-player__viz {
+    display: block;
+    height: 14px;
+    width: 60px;
+}
+
 .radio-player__station-name {
     font-size: 14px;
     font-weight: 600;