docs: add comprehensive CONTRIBUTING guide for mobile contributors

[adesuwa-tech]

Summary

Adds a complete CONTRIBUTING.md to help new mobile contributors get up and running quickly with the Expo app.

Changes

• Prerequisites — Node.js v18+, npm v9+, Expo CLI, Expo Go (and optional simulator setup)
• Installation — step-by-step clone, npm install --legacy-peer-deps, and .env setup
• Running the app — npm start plus android, ios, and web shortcuts
• Project structure — annotated directory tree covering app/, src/, docs/, and test folders
• Testing — npm test / npm run test:watch, UI testing expectations (jest-expo + RNTL), mocking patterns
• UI & Design guidelines — link to design system, token import rules, dark-only palette note
• Accessibility — checklist highlights, touch targets, contrast, screen reader annotations
• Security — key handling rules, no-commit reminder, link to Security Guide
• PR checklist — branch naming, commit message format, push, and review steps

What was tested

• Verified all linked docs (docs/design-system.md, docs/accessibility.md, docs/security.md, docs/storage.md) exist in the repo.
• Confirmed npm install --legacy-peer-deps and npm start / npm test scripts match package.json.
• Markdown renders cleanly with correct headings, table, and code blocks.

Closes

Closes Axionvera#3