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/locales/en.json

@@ -259,8 +259,13 @@
   "links.msg.load_failed": "Failed to load link details",
   "links.confirm.delete": "Are you sure you want to delete the link \"{name}\"?",
   "links.confirm.unsaved": "You have unsaved changes. Are you sure you want to discard them?",
-  "footer.created_by": "Created by",
-  "footer.source_code": "Source Code",
+  "about.button_title": "About",
+  "about.title": "About",
+  "about.created_by": "Created by",
+  "about.email": "Email",
+  "about.repository": "Repository",
+  "about.source_code": "Source Code",
+  "dialog.close": "Close",
   "update.available": "Update available: v{version}",
   "update.view_release": "View Release"
 }