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>

inc/admin-page.php

@@ -41,6 +41,10 @@ function radio_render_main_page() {
                 <div class="radio-player" data-radio-surface="main">
 
                     <div class="radio-player__now">
+                        <span class="radio-player__indicator" aria-hidden="true">
+                            <span class="radio-player__bars"><span></span><span></span><span></span><span></span></span>
+                            <canvas class="radio-player__viz" data-radio-viz hidden></canvas>
+                        </span>
                         <span class="radio-player__label"><?php esc_html_e( 'Now Playing', 'radio' ); ?></span>
                         <span class="radio-player__station-name" data-radio-name><?php echo esc_html( $station['name'] ); ?></span>
                         <span class="radio-player__station-genre" data-radio-genre><?php echo esc_html( $station['genre'] ); ?></span>
@@ -82,7 +86,7 @@ function radio_render_main_page() {
 
                     <div class="radio-player__error" data-radio-error hidden></div>
 
-                    <audio data-radio-audio preload="none"></audio>
+                    <audio data-radio-audio preload="none" crossorigin="anonymous"></audio>
                 </div>
             </div>
         </div>