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>

inc/settings.php

@@ -82,7 +82,7 @@ function radio_render_settings_page() {
                         <label for="default_volume"><?php esc_html_e( 'Default volume', 'radio' ); ?></label>
                     </th>
                     <td>
-                        <input type="range" id="default_volume" name="default_volume" min="0" max="100" value="<?php echo esc_attr( (int) round( $state['volume'] * 100 ) ); ?>" oninput="document.getElementById('default_volume_label').textContent = this.value + '%'">
+                        <input type="range" id="default_volume" name="default_volume" min="0" max="100" value="<?php echo esc_attr( (int) round( $state['volume'] * 100 ) ); ?>" aria-describedby="default_volume_label">
                         <span id="default_volume_label"><?php echo esc_html( (int) round( $state['volume'] * 100 ) ); ?>%</span>
                     </td>
                 </tr>