Accessibility and clarity fixes for the sign-in screen.

• The "Continue with Google" and "Sign in" buttons now explain why they are
  unavailable: pressing one without accepting the Terms and Privacy Policy
  highlights the checkbox and moves focus to it, instead of doing nothing.
• Those buttons are now announced by screen readers while unavailable, rather
  than being skipped entirely.
• Fixed a popup sizing issue affecting Chromium-based builds, where the window
  could remain widened after a failed sign-in.

No changes to encryption, permissions, or how secrets are handled.

Fixed a privacy issue: signing out now clears the secret list cached in this
browser. Previously the list — secret IDs and your own labels — stayed on the
device after sign-out and was visible to the next person signing in on the
same browser profile. No secret content or decryption keys were ever stored;
those exist only in the share link. Signing out also clears any unsent draft.