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>

radio.php

@@ -5,7 +5,7 @@
  * Plugin Name: Radio
  * Plugin URI:  https://icanhelp.ie/radio
  * Description: A small, focused, free radio player for your WordPress admin. 44 SomaFM stations grouped by 10 genres — ambient, electronic, lounge, rock, metal, jazz, world, reggae, holiday, specials. Plays via HTML5 audio; volume + station choice persist per-user.
- * Version:     0.3.2
+ * Version:     0.4.0
  * Requires at least: 5.0
  * Requires PHP: 7.4
  * Author:      David Keane
@@ -20,7 +20,7 @@
 if ( ! defined( 'ABSPATH' ) ) { exit; }
 
 // Plugin coordinates.
-if ( ! defined( 'RADIO_VERSION' ) )   { define( 'RADIO_VERSION',   '0.3.2' ); }
+if ( ! defined( 'RADIO_VERSION' ) )   { define( 'RADIO_VERSION',   '0.4.0' ); }
 if ( ! defined( 'RADIO_FILE' ) )      { define( 'RADIO_FILE',      __FILE__ ); }
 if ( ! defined( 'RADIO_PATH' ) )      { define( 'RADIO_PATH',      plugin_dir_path( __FILE__ ) ); }
 if ( ! defined( 'RADIO_URL' ) )       { define( 'RADIO_URL',       plugin_dir_url( __FILE__ ) ); }