Improve screen-reader support

Targeting issues raised in #341 and as part of #84.

• Heading structure: > not supported by upstream Compose (https://github.com/JetBrains/compose-jb/issues/2119)
  • "Welcome to Briar" on login and main screen (no chats)
  • contact name in chat screen
  • "Settings" on SettingsScreen
• Image(Button) contentDescription:
  • Briar logo on login > only of decorative nature, automatically skipped over by VoiceOver
  • back and info button on login > !223 (merged) and info button already "About Briar Desktop" on main
  • menu button in chat screen > already "Show Contact Menu" on main
  • add contact button without chats > fixed on main
  • attachment button in chat screen > fixed on main
• Missing context:
  • show password buttons on login/change password > automatically grouped on macOS/VoiceOver
  • labels for text fields (registration screen) > !225 (merged)
• Missing list grouping:
  • contact list > !218 (merged)
  • settings as list > !224 (merged)
• Missing state information (expanded/collapsed): Dropdowns/Pop-Ups not supported by Compose https://github.com/JetBrains/compose-jb/issues/2185
  • menu button in chat screen
  • theme/language settings, also dynamic changes
• Misc:
  • error message on login (not read out loud) > upstream bug: https://github.com/JetBrains/compose-jb/issues/2277
  • about dialog (no exit button, table not marked as such, email is not link) > !221 (merged)
  • message count/online status in contact list > !218 (merged)
  • "keyboard trap" in compose message text field (tab button is stuck in text field) > !222 (merged)
  • Missing landmarks on MainScreen to convey structure to screen-reader user > not supported by Compose
  • Keyboard focus does not go to close button on add contact dialog > probably missing OS-functionality on Ubuntu/Orca which is anyhow not supported, confirmed to work on macOS with VoiceOver as expected
  • Dropdown not marked as such (settings) > Dropdowns/Pop-Ups not supported by Compose https://github.com/JetBrains/compose-jb/issues/2185
  • Briar link name/label on Add Contact dialog > !230 (merged)
  • Add attachment button not keyboard-focusable on macOS > !222 (merged)