feat(0.6.3): discreet "buy me a coffee" support link

Tiny footer line inside the Updates panel on Settings + matching line
at the bottom of the Credits + thanks card on About. Pulled from a
single RADIO_SUPPORT_URL constant so both stay in sync. Conditional
render — if the constant is empty, the link is silently hidden, so
forks can strip funding with one line.

Design intent: muted (12px, admin-theme-coloured, subtle top divider).
Reads as housekeeping not a sales pitch — no yellow BMC brand chrome.
Dark-theme divider override so it stays subtle on the dark surface.

Files: radio.php (version, RADIO_SUPPORT_URL constant default
https://buymeacoffee.com/davidtkeane), inc/updater.php (link in
Updates panel), inc/about.php (link in Credits + thanks card; rotate
v0.6.3 → latest, v0.6.2 → earlier list), assets/css/radio.css
(.radio-support-link + dark-theme override), CHANGELOG.md.

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

assets/css/radio.css

@@ -297,8 +297,25 @@
     font-size: 13px;
 }
 
+/* v0.6.3: Discreet "buy me a coffee" support line. Rendered in two
+   spots from the single RADIO_SUPPORT_URL constant — bottom of the
+   Updates panel in Settings, and bottom of the Credits + thanks card
+   on About. Muted styling deliberate: housekeeping not sales pitch. */
+.radio-support-link {
+    display: block;
+    margin-top: 12px;
+    padding-top: 10px;
+    border-top: 1px solid #f0f0f1;
+    font-size: 12px;
+    color: var(--wp-admin-theme-color, #2271b1);
+    text-decoration: none;
+}
+.radio-support-link::before { content: '☕  '; opacity: 0.8; }
+.radio-support-link:hover { text-decoration: underline; }
+.radio-theme-dark .radio-support-link { border-top-color: #3c434a; }
+
 /* Small grey pill that follows the Settings page H1 — at-a-glance
-   confirmation of the version you are running (v0.6.2). */
+   confirmation of the version you are running. */
 .radio-version-badge {
     display: inline-block;
     margin-left: 8px;