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/devices/LocalBackup.tsx

@@ -70,6 +70,7 @@ function ExportKeys() {
                   <PasswordInput
                     ref={passRef}
                     name="passwordInput"
+                    aria-label="New backup password"
                     size="400"
                     variant="Secondary"
                     radii="300"
@@ -85,6 +86,7 @@ function ExportKeys() {
                     ref={confPassRef}
                     style={{ color: match ? undefined : color.Critical.Main }}
                     name="confirmPasswordInput"
+                    aria-label="Confirm backup password"
                     size="400"
                     variant="Secondary"
                     radii="300"
@@ -212,6 +214,7 @@ function ImportKeys({ file, onDone }: ImportKeysProps) {
             <Text size="L400">Password</Text>
             <PasswordInput
               name="passwordInput"
+              aria-label="Backup password"
               size="400"
               variant="Secondary"
               radii="300"