New features + search/a11y polish.

• 4-step welcome sheet on first launch.
• Optional biometric app lock (off by default, on-device).
• Export any note as PDF from the share menu.
• Widget shows your recent notes; tap to open.
• Search now tokenizes Korean / Japanese / Chinese correctly.
• Japanese (ja) UI translation.
• Settings: license / GitHub / F-Droid links.
• Predictive back gesture (Android 13+).
• Section headers announce as headings to TalkBack.
