feat(a11y): form input labels (H-6), remaining button labels (C-1)

H-6: aria-label on all form inputs missing accessible names:
  - Login: username, password (already done)
  - Register: username, password, confirm, token, email
  - Password reset: email, new password, confirm password
  - Settings: display name, user ID to ignore, keyword, page zoom,
    date format, device name, backup passwords (new/confirm/restore)
  - Auth: server URL picker input
C-1: additional icon buttons:
  - RoomInput: toolbar toggle (aria-pressed + label)
  - Lobby/Members: scroll to top, toggle member list
  - UIAFlowOverlay: cancel authentication
  - BackupRestore: backup options menu
  - UrlPreview: previous/next preview buttons
  - RoomPacks: undo remove/remove pack buttons
  - RoomViewHeader: start call, member list toggle
  - ServerPicker: change server button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/settings/general/General.tsx

@@ -293,6 +293,7 @@ function PageZoomInput() {
   return (
     <Input
       style={{ width: toRem(100) }}
+      aria-label="Page zoom percentage"
       variant={pageZoom === parseInt(currentZoom, 10) ? 'Secondary' : 'Success'}
       size="300"
       radii="300"
@@ -617,6 +618,7 @@ function CustomDateFormat({ value, onChange }: CustomDateFormatProps) {
           <Input
             required
             name="customDateFormatInput"
+            aria-label="Custom date format"
             value={dateFormatCustom}
             onChange={handleChange}
             maxLength={16}