feat(onboarding): install prompt, push notification prompt, and schedule tip

[5sansiva]

Summary

Chained onboarding flow for hackers: install PWA → enable push notifications → learn to browse the schedule. Shown one step at a time via toast instead of all at once.

What's new

• Install prompt (InstallPromptHost, shared/install/) — detects platform via useInstallPrompt. iOS shows manual "Tap Share → Add to Home Screen" steps (no native install API on Safari); Android/Chromium gets a real "Install" button wired to beforeinstallprompt. Shown once per browser (install-prompted-v1).
• Manual install trigger — added "Install app" row to hacker Profile settings to re-trigger the flow after the initial toast is dismissed.
• Push prompt (PushPromptHost, shared/push/usePushPrompt.ts) — fires once installed/standalone and permission is unset. Gated by push-prompted-v1.
• Schedule tip toast (ScheduleTipHost, shared/schedule-tip/) — one-time tip on browsing the schedule (tap events for details, use the filter icon). Chained after the push prompt resolves via a push-prompt-settled event, and only shown if a schedule is configured (getScheduleDateRange().configured). Mounted globally in HackerLayout.
• Toast UI polish — moved toaster to top-center (bottom conflicted with the mobile nav bar); fixed a text-wrapping bug in iOS install instructions.

Notes

• No backend changes.
• go.mod/go.sum bump for go.mozilla.org/pkcs7 (unrelated, picked up incidentally).

Testing

• npm run build, npm run lint, npm run format:check, tsc -b --noEmit
• go vet ./...
• Manually verified full chain (install → push → schedule tip) on a real device — requires HTTPS tunnel since push needs a secure context and SuperTokens is localhost-locked in dev