feat: Android download page, analysis retry and a collapsible search

- /android is a public page with the latest APK, the server address to copy
  and three install steps; linked from the sign-in screen and the profile menu
- failed audio analysis can be retried from the library page, which also shows
  the most common error; the analyzer no longer burns attempts on files that
  are missing because the disk is not mounted
- the top bar search folds into a round button and slides open on click, "/"
  or Ctrl+K

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mobile.md

@@ -82,6 +82,8 @@
 
 ## Самообновление
 
+Первая установка — публичная страница `/android` веб-клиента: последний APK, адрес сервера и инструкция.
+
 Приложение распространяется только через Gitea Releases (не Google Play).
 
 **Поток:**