feat(onboarding): add mission and lifecycle support prompts

[markm39]

Summary

• replace the first-run flow with a mission-only three-slide introduction and bespoke transparent 3D artwork
• keep onboarding locally persisted and replayable from the single heart entry in the library header
• add a persistent support sheet for Discord, native store ratings, GitHub stars, mission, privacy, terms, and support
• show the Discord community invitation after three uniquely saved notes and persist join or dismissal exactly once
• request the native store review after five uniquely saved notes, seven days of use, and one day after the community prompt, at most once per app version
• serialize lifecycle state updates and recover interrupted community prompts on the next launch
• constrain sheets to safe areas and support compact phones, iPads, landscape, scrolling, and maximum Dynamic Type

Verification

• npm run test:lifecycle (7 passing policy tests)
• npm run typecheck
• npx expo export --platform android --output-dir /tmp/opennotes-android-export-final
• native iOS development build succeeded and installed on an iPhone 17 Pro simulator
• verified onboarding, header support sheet, third-note Discord prompt, dismissal persistence, interrupted-prompt recovery, native review presentation, and once-per-version review persistence
• visually verified 375x667 compact phone, 402x874 notched phone, 834x1210 iPad portrait, iPad landscape, and maximum Dynamic Type layouts
• git diff --check and trailing-whitespace scan passed

Review gates

• simplify: passed. app/index.tsx is 498 lines after extracting lifecycle support logic. The 903-line note editor predates this change; this PR makes only a surgical successful-save instrumentation change there, and splitting that editor in this feature would increase regression risk.
• security: the feature review found no authorization, injection, secret, sensitive-data, or unsafe-link issues. Community navigation uses the fixed Discord invite https://discord.gg/VWKmgMgYu.
• dependency audit: npm audit --omit=dev --audit-level=high reports 30 pre-existing transitive Expo/toolchain advisories, including 2 critical findings. Available forced remediation upgrades Expo across a breaking SDK boundary, so this PR must remain open and must not be self-merged until that repository-wide dependency gate is resolved.