feat(0.3.0): dark theme, mute, media keys, current-track + 2nd-look fixes

Closes the gaps from a UI review of v0.2.0.

Added
- Mute toggle: speaker icon is now a button; remembers prior volume.
- MediaSession API: OS media keys / headphone buttons / lock-screen
  widget play/pause the radio. Metadata exposes station + SomaFM + genre.
- Current-track display: polls https://somafm.com/songs/{code}.json every
  30s while playing; shown as `♪ Title — Artist` under the description.
  Best-effort — silently hidden if CORS-blocked / unreachable.

Fixed (2nd-look)
- Dark theme now actually renders. v0.2.0 saved the dropdown but had no
  CSS — add `admin_body_class` filter + `radio-theme-{auto,light,dark}`
  CSS for the player + about-cards. `auto` follows OS prefers-color-scheme.
- Settings-page volume slider: removed inline `oninput`; wired in radio.js
  via `bindSettingsSlider()`. Cleaner under strict CSP.
- Save errors surface as a transient notice instead of being swallowed.
- Gitea changelog URL moved into `RADIO_GITEA_URL` constant.
- Genre badge restyled as an inline pill (was using `margin-left: auto`
  which wrapped poorly on narrow widget widths).

Files
- radio.php (version, constant, strings, body-class filter)
- inc/about.php (use constant, add 0.3.0 history entry)
- inc/settings.php (drop inline oninput)
- inc/admin-page.php + inc/dashboard-widget.php (mute button, track slot)
- assets/css/radio.css (pill, mute, track, dark-theme rules)
- assets/js/radio.js (rewrite: mute, MediaSession, track polling,
  settings slider, save-error surfacing)

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:     0.2.0
+ * Version:     0.3.0
  * Requires at least: 5.0
  * Requires PHP: 7.4
  * Author:      David Keane
@@ -20,11 +20,12 @@
 if ( ! defined( 'ABSPATH' ) ) { exit; }
 
 // Plugin coordinates.
-if ( ! defined( 'RADIO_VERSION' ) )  { define( 'RADIO_VERSION',  '0.2.0' ); }
-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__ ) ); }
-if ( ! defined( 'RADIO_BASENAME' ) ) { define( 'RADIO_BASENAME', plugin_basename( __FILE__ ) ); }
+if ( ! defined( 'RADIO_VERSION' ) )   { define( 'RADIO_VERSION',   '0.3.0' ); }
+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__ ) ); }
+if ( ! defined( 'RADIO_BASENAME' ) )  { define( 'RADIO_BASENAME',  plugin_basename( __FILE__ ) ); }
+if ( ! defined( 'RADIO_GITEA_URL' ) ) { define( 'RADIO_GITEA_URL', 'https://git.davidtkeane.com/ranger/a-radio' ); }
 
 // Includes — each file owns one concern.
 require_once RADIO_PATH . 'inc/stations.php';         // the 44-station array + genre helpers
@@ -124,6 +125,9 @@ function radio_enqueue_admin_assets( $hook ) {
             'pause'      => __( 'Pause', 'radio' ),
             'loading'    => __( 'Loading…', 'radio' ),
             'error'      => __( 'Stream error — try another station.', 'radio' ),
+            'saveError'  => __( 'Preferences not saved — check your connection.', 'radio' ),
+            'mute'       => __( 'Mute', 'radio' ),
+            'unmute'     => __( 'Unmute', 'radio' ),
             'nowPlaying' => __( 'Now Playing', 'radio' ),
             'volume'     => __( 'Volume', 'radio' ),
             'station'    => __( 'Station', 'radio' ),
@@ -158,6 +162,21 @@ function radio_ajax_save_state() {
     wp_send_json_success( radio_get_state() );
 }
 
+/**
+ * Surface the user's theme choice (auto/light/dark) to CSS as a body
+ * class. `radio-theme-dark` forces dark; `radio-theme-auto` lets the
+ * CSS respect `prefers-color-scheme`; `radio-theme-light` is the
+ * default no-op.
+ */
+add_filter( 'admin_body_class', 'radio_admin_body_class' );
+function radio_admin_body_class( $classes ) {
+    if ( ! is_user_logged_in() ) { return $classes; }
+    $state = radio_get_state();
+    $theme = isset( $state['theme'] ) ? $state['theme'] : 'auto';
+    if ( ! in_array( $theme, array( 'auto', 'light', 'dark' ), true ) ) { $theme = 'auto'; }
+    return $classes . ' radio-theme-' . $theme;
+}
+
 /**
  * Activation hook: ensure the version option is set so the updater can
  * track it.