Update mobile/README.md for development builds — Expo Go no longer runs the app

Description

ADR 0016 (lands with #128) makes development builds mandatory: useSQLCipher is a build-level flag, so SQLCipher requires npx expo prebuild and a dev build, and Expo Go can no longer run this app.

mobile/README.md was not updated to match, so the moment #128 merges the onboarding instructions tell contributors to do something that will fail:

• mobile/README.md:13 — lists Expo Go as a prerequisite
• mobile/README.md:31 — "scan the QR code with Expo Go"
• mobile/README.md:53 — "Expo Go still runs the app on a…"

0016 calls this out directly: "Contributor onboarding changes and the docs have to change with it… the first-run cost (a native build) needs saying up front rather than discovered." It also notes this lands on the same people 0008 designed a gentle first ticket for, so the setup instructions matter more here than usual.

ADR 0017 adds a second doc gap: the journal database refuses to open on web by design, so npm run web is the landing page and not a preview of the app. A contributor who runs it and sees something render will reasonably assume otherwise.

Scope is mobile/README.md and any sibling contributor docs — no code changes.

Acceptance Criteria

• Expo Go removed as a way to run the app; replaced with the npx expo prebuild + development-build path
• First-run cost (a native build, and that it is slow the first time) stated up front, not buried
• Prerequisites corrected — Xcode/Android Studio requirements for a dev build, and the required Node version (engines is >=20.19.4)
• npm run web documented as the landing/marketing surface only, per 0017 — explicitly not an app preview
• A contributor with a clean checkout can follow the README start to finish and reach a running app
• The feature/s being implemented are covered by unit tests - If not, create tests for them on this ticket (docs-only; no tests expected)

Additional Info and Resources

• Source: ADR 0016 (Consequences) and ADR 0017 (Consequences), both in #128
• 0016 also notes demoing to non-technical stakeholders no longer works via Expo Go — a shared development build or the web landing page is the substitute, worth a line in the README
• CI is unaffected (lint/typecheck/jest need no native build), so this is purely a local-development docs problem

QA

• On a clean clone, follow the README exactly on macOS (iOS) and confirm the app runs
• Repeat on Linux/Windows (Android) and confirm the instructions hold
• Confirm nothing in the README still implies Expo Go or npm run web will show the journal