feat(0.6.0): pop-out mini-player — continuous background play across admin nav

Until v0.5.0 the audio cut every time you navigated between WP admin
pages — every navigation is a full page reload, which destroys the
<audio> element. v0.6.0 fixes the background-music use case by letting
you pop the player out into a separate browser window that persists
across the parent tab's navigation.

Pop-out window
  - Small "↗ Pop out" button beside the Play button on both the main
    Radio page and the Dashboard widget.
  - Opens a 380×560 standalone window via window.open() with a window
    name of `radio_popout` so a second click re-focuses the existing
    popup rather than spawning a new one.
  - Popup renders at admin-post.php?action=radio_popout&play=1 — a new
    admin_post_radio_popout hook outputs a full standalone HTML page
    (custom DOCTYPE / head / body, no WP admin chrome).
  - Theme follows radio_state['theme'] via body class radio-theme-{...}.
  - Includes the full player (now-playing block with dancing bars +
    Web Audio visualizer, play, mute, volume, station dropdown, error
    slot). Close button calls window.close().

Auto-resume + single-stream invariant
  - cfg.autoPlay is true when the popup URL carries &play=1. radio.js
    auto-calls audio.play() 200 ms after init. Same-origin user-gesture
    popups are exempt from autoplay-blocking on every modern browser.
  - On pop-out click, every other audio surface in the parent tab is
    paused so there is only ever one stream playing.

State stays in sync
  - Popup uses the SAME radio_save_state AJAX + track-logging endpoint.
    Station / volume changes persist to user_meta; track history keeps
    accumulating from whichever surface is playing.

Edge cases
  - Inside the popup, cfg.popoutUrl is '' so bindPopOut hides any
    Pop-out button it finds (would be infinite otherwise).
  - Popup blocked by browser → clear alert tells the user to allow
    popups for this site.

Files
  - radio.php (version, popoutUrl in localized config, admin_post
    handler + full standalone HTML renderer)
  - inc/admin-page.php + inc/dashboard-widget.php (Pop-out button
    beside Play)
  - assets/css/radio.css (.radio-player__popout styling)
  - assets/js/radio.js (bindPopOut function; autoPlay branch in
    bindPlayer)
  - inc/about.php + CHANGELOG.md (history entries)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

inc/about.php

@@ -48,7 +48,11 @@ function radio_render_about_page() {
                 <h2><?php esc_html_e( 'Version history', 'radio' ); ?></h2>
                 <ul>
                     <li>
-                        <span class="ver">v0.5.0</span> &mdash; 29 May 2026 <span class="latest">latest</span><br>
+                        <span class="ver">v0.6.0</span> &mdash; 30 May 2026 <span class="latest">latest</span><br>
+                        <?php esc_html_e( 'Pop-out mini-player. A small "↗ Pop out" button next to Play opens the player in its own 380×560 window. The popup persists across the main tab\'s navigation, so the music keeps playing while you click around Plugins / Posts / Users. The popup auto-resumes where the main tab left off, pauses the main-tab audio so you don\'t double-stream, and uses the same state + history + favourites as everywhere else.', 'radio' ); ?>
+                    </li>
+                    <li>
+                        <span class="ver">v0.5.0</span> &mdash; 29 May 2026<br>
                         <?php esc_html_e( 'Track history + favourites. Every track that scrolls past in the player is now quietly logged to your personal Radio → History page (capped at 500), with a star toggle to keep the good ones forever in a separate Favourites tab. Each row has four search links (Spotify / YouTube / Apple Music / Bandcamp) so you can find that track on whichever service you use. Filter by station, search by artist or title, clear history with a button. Per-user, nothing leaves your site.', 'radio' ); ?>
                     </li>
                     <li>

inc/admin-page.php

@@ -58,6 +58,10 @@ function radio_render_main_page() {
                             <span data-radio-play-label><?php esc_html_e( 'Play', 'radio' ); ?></span>
                         </button>
 
+                        <button type="button" class="button radio-player__popout" data-radio-popout title="<?php esc_attr_e( 'Open in a pop-out window — keeps playing while you navigate the admin', 'radio' ); ?>">
+                            <span aria-hidden="true">↗</span> <?php esc_html_e( 'Pop out', 'radio' ); ?>
+                        </button>
+
                         <div class="radio-player__volume">
                             <button type="button" class="radio-player__mute" data-radio-mute aria-label="<?php esc_attr_e( 'Mute', 'radio' ); ?>">
                                 <span class="dashicons dashicons-controls-volumeon" aria-hidden="true"></span>

inc/dashboard-widget.php

@@ -51,6 +51,10 @@ function radio_render_dashboard_widget() {
                 <span data-radio-play-label><?php esc_html_e( 'Play', 'radio' ); ?></span>
             </button>
 
+            <button type="button" class="button radio-player__popout" data-radio-popout title="<?php esc_attr_e( 'Open in a pop-out window — keeps playing while you navigate', 'radio' ); ?>">
+                <span aria-hidden="true">↗</span> <?php esc_html_e( 'Pop out', 'radio' ); ?>
+            </button>
+
             <div class="radio-player__volume">
                 <button type="button" class="radio-player__mute" data-radio-mute aria-label="<?php esc_attr_e( 'Mute', 'radio' ); ?>">
                     <span class="dashicons dashicons-controls-volumeon" aria-hidden="true"></span>