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>

assets/js/radio.js

@@ -412,6 +412,44 @@
         }
 
         bindMute(player, audio, volumeIn, volPctEl);
+        bindPopOut(player);
+
+        // v0.6.0: popout auto-resume — when opened with ?play=1, immediately
+        // pick up where the main tab left off. Same-origin user-gesture popups
+        // are exempt from autoplay blocking on every modern browser.
+        if (cfg.autoPlay) {
+            setTimeout(function () {
+                if (audio.paused) { audio.play().catch(function () { /* autoplay denied — user just clicks play */ }); }
+            }, 200);
+        }
+    }
+
+    /** v0.6.0: Pop-out window button. Opens a 380×560 standalone window
+     *  with just the player chrome (no WP admin), via admin-post.php?
+     *  action=radio_popout&play=1. The popup persists across main-tab
+     *  navigation so background music doesn't cut when you move around
+     *  the WP admin. Pauses every other audio surface in this tab so
+     *  there's only one stream playing at a time. */
+    function bindPopOut(player) {
+        var btn = player.querySelector('[data-radio-popout]');
+        if (!btn) { return; }
+        if (!cfg.popoutUrl) { btn.style.display = 'none'; return; } // already in popout
+        btn.addEventListener('click', function () {
+            var w = window.open(
+                cfg.popoutUrl + '&play=1',
+                'radio_popout',
+                'width=380,height=560,resizable=yes,scrollbars=no,toolbar=no,location=no,menubar=no,status=no'
+            );
+            if (!w) {
+                window.alert('Pop-out blocked by the browser. Allow popups for this site, then try again.');
+                return;
+            }
+            w.focus();
+            // Pause every audio surface in this tab — the popup is the new player.
+            document.querySelectorAll('[data-radio-audio]').forEach(function (a) {
+                if (!a.paused) { a.pause(); }
+            });
+        });
     }
 
     /** Keep all .radio-player surfaces on the same station. */