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.

inc/settings.php

@@ -3,8 +3,7 @@
  * Radio — Settings page.
  *
  * Lets the user pick default station + volume + theme + dashboard
- * widget opt-out. Renders the Updates panel from `updater.php` at the
- * bottom.
+ * widget opt-out.
  */
 
 if ( ! defined( 'ABSPATH' ) ) { exit; }
@@ -121,17 +120,6 @@ function radio_render_settings_page() {
 
             <?php submit_button( __( 'Save Changes', 'a-radio' ), 'primary', 'radio_settings_submit' ); ?>
         </form>
-
-        <?php
-        // Updates panel — only manage_options users see it, and only when
-        // the custom updater is active (i.e. self-hosted Gitea install,
-        // NOT when distributed via wp.org where wp.org handles updates).
-        // The function is undefined when updater.php short-circuited at
-        // load time per wp.org guideline 8 — hence the function_exists() check.
-        if ( current_user_can( 'manage_options' ) && function_exists( 'radio_render_updates_panel' ) ) {
-            radio_render_updates_panel();
-        }
-        ?>
     </div>
     <?php
 }