v0.7.4 — wp.org submission cleanup: remove updater + add LICENSE

Walks back v0.7.3's Update URI guard pattern. Plugin Check raised
`plugin_updater_detected` on the v0.7.3 build:

    "Including An Update Checker / Changing Updates functionality.
     Plugin Updater detected. Use of the Update URI header is not
     allowed in plugins hosted on WordPress.org."

PCP scans the source as-shipped, not as-distributed, so the v0.7.3
build-time `sed` strip never had a chance to run before the scan.
The simpler correct answer is to delete the custom updater entirely
and rely on wp.org as the canonical update channel once accepted.

Removed:
* `inc/updater.php` — recoverable from git history at tag v0.7.3 if
  ever needed for a non-wp.org distribution.
* `Update URI:` header line in `radio.php` (plus the NOTE block).
* `require_once RADIO_PATH . 'inc/updater.php';` in `radio.php`.
* Updates panel render + `function_exists()` guard in `inc/settings.php`.
* "Self-hosted update checker" line in `README.md`.
* "Self-hosted updater" bullet in `readme.txt` Privacy section.

Added (GPL declaration loop closed):
* `LICENSE` — verbatim canonical GPL v2 text (338 lines from
  gnu.org/licenses/gpl-2.0.txt).
* GPL header block in `assets/css/radio.css`.
* GPL header block in `assets/js/radio.js` (original module overview
  preserved verbatim below the license header).
* GPL header block in `radio.php` docblock alongside the existing
  `License:` / `License URI:` fields.

Migration: existing Gitea-installed copies of v0.7.3 or earlier
become orphaned of auto-updates after this lands on them (the
updater code is gone, so nothing advertises newer versions back).
Recommended path is to uninstall + reinstall from wp.org once the
plugin is accepted. No data loss — station + volume + theme +
history + favourites all live in user_meta.

No user-visible behaviour changes for the player itself. Only the
small `Updates` panel that sat at the bottom of Radio → Settings
is gone.

assets/js/radio.js

@@ -1,5 +1,15 @@
-/**
- * Radio — vanilla JS audio controller.
+/*!
+ * RangerHQ Radio — vanilla JS audio controller.
+ *
+ * Copyright (C) 2026 David Keane
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version. See LICENSE in the plugin root
+ * for the full text, or visit https://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * --- module overview ---
  *
  * Binds to every `.radio-player` element on the page (dashboard widget
  * + main page can co-exist; both share one logical audio session per