feat: add About and Help screens and custom drawer content

Stacked on !7

Summary

About CDLI and Help & FAQ screens, reachable from a custom side drawer

Changes

• Custom drawer - header with CDLI wordmark + app version badge, DrawerItemList for the rows, black "Visit cdli.earth" button and "Open Source • GSoC 2026" pinned to the bottom
• About CDLI - app icon, mission card, team, links to cdli.earth and the CDLI GitLab, app version
• Help & FAQ - FAQ / Send Feedback tabs, single-open accordion (5 entries), Contact Support and Send Feedback via mailto: -> consistent with CDLI's existing email workflow
• lib/constants.ts - URLs, support email, mission text, team, FAQ content in one place
• Version read from app.json via expo-constants -> bump once, both places follow

Notes

• Both mail actions go to cdli-support@ames.ox.ac.uk with different subject lines

Screenshots