fix(0.3.1): My Radio layout polish + drop dark-auto

Real-screen review of v0.3.0 surfaced a contrast bug and several layout
issues. Quick patch.

- Drop @media prefers-color-scheme dark for theme=auto. WP admin has
  no native dark mode, so OS-dark made our light text sit on the still-
  white WP postbox = unreadable. `auto` now behaves as light; `dark`
  remains as an explicit choice.
- theme=dark now actually reads: the player surface itself goes dark
  (#1d2327 bg + subtle border + padding) so the light text has somewhere
  to land instead of fighting the white WP postbox.
- .radio-wrap max-width 880px — player no longer stretches edge-to-edge.
- Drop the .radio-intro max-width:720px cap so the intro fits one line.
- Volume slider fixed at flex:0 0 auto / width:220px — % label sits next
  to the slider instead of pinned to the far edge.
- Station dropdown capped at 360px (was width:100%).
- Play-button dashicon shrunk 18px → 14px so it sits on the button-text
  baseline instead of looking like a second row.

Files: radio.php, assets/css/radio.css, inc/about.php, CHANGELOG.md.

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:     0.3.0
+ * Version:     0.3.1
  * 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',   '0.3.0' ); }
+if ( ! defined( 'RADIO_VERSION' ) )   { define( 'RADIO_VERSION',   '0.3.1' ); }
 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__ ) ); }