PWA install prompt (smart timing)

Priority: P1
Estimate: 0.5 day

- Do not prompt on first visit
- Prompt after first completed session
- Show visual guide
- Dismiss for 7 days
- Track install rate via PostHog

## Acceptance criteria:
- [ ] Prompt appears after first session completion only
- [ ] Guide is visual and bilingual
- [ ] Dismiss is persistent
- [ ] Install rate tracked as analytics event

