docs: refresh README structure and add CONTRIBUTING guide

[AnnieIj]

Closes #143

Summary

• Updated README project layout to reflect the real src/ tree (pages/, components/, hooks/, data/, utils/, ThemeContext.tsx)
• README routes table lists all 7 actual routes; removed stale 'tabs' wording
• Scripts table matches package.json exactly (dev, build, preview)
• Added CONTRIBUTING.md with setup, branch naming, design-system token rules, accessibility expectations, and note on upcoming Vitest integration
• All relative doc links resolve (docs/UI-Design-System.md, CONTRIBUTING.md)

What was tested

• npm run build — pre-existing TS errors unrelated to this change; docs only
• All relative links verified to exist on disk

Notes

Docs-only change — no code modified.

[greatest0fallt1me]

the readme had drifted a fair bit from the actual src/ tree so this cleanup is welcome, and we were missing a real CONTRIBUTING guide entirely. rebased it on top of the format/constants change that just landed and kept both. thanks for sorting this out 📝