ui(player): replace footer with About dialog + reclaim dead space

- Move colophon (credit/email/source link) from sticky footer into
  a dedicated About dialog, opened from a new header button
- Drop ~64px of bottom container padding now that the footer is gone
- Loosen vinyl-stage aspect-ratio (1:1 -> 1:0.85) so the disc no
  longer leaves a tall empty band below the sleeve
- Switch tonearm height: 36% to aspect-ratio: 1 to keep proportions
  consistent across the new stage ratio
- Add about.* / dialog.close i18n keys for EN and RU
- Add vinyl-variants-mockup.html as next design reference target

media_server/static/js/core.js

@@ -397,6 +397,16 @@ export function closeDialog(dialog) {
     }, { once: true });
 }
 
+export function showAboutDialog() {
+    const dialog = document.getElementById('aboutDialog');
+    if (dialog) dialog.showModal();
+}
+
+export function closeAboutDialog() {
+    const dialog = document.getElementById('aboutDialog');
+    if (dialog) closeDialog(dialog);
+}
+
 export function showConfirm(message) {
     return new Promise((resolve) => {
         const dialog = document.getElementById('confirmDialog');