docs: add PROJECT_ARCHITECTURE.md (project overview & architecture)

[arnavgoel17]

Summary

Closes #199.

Adds a dedicated PROJECT_ARCHITECTURE.md so new contributors can understand how EduBridge is organized and how the pieces interact, and links it from the README.

What's included

• High-level overview of the app (static multi-page frontend + Flask chat backend + OpenAI)
• Architecture diagram — both ASCII (always visible) and Mermaid (renders on GitHub)
• Repository structure with per-folder descriptions + a page → scripts inventory table
• Data flow / execution pipeline — page load & auth state, authentication, content grids, quizzes, admin dashboard, and the hybrid (remote-first → local fallback) chatbot
• Backend reference — the /chat endpoint contract and how to run it
• Component dependencies + the shared localStorage schema table
• Contributor notes flagging the legacy server/ Node experiments and the stray backend/app duplicate, so newcomers aren't confused about which backend is live

Everything is derived from reading the actual code (scripts/*.js, backend/app.py, server/*.js, and the HTML pages), so it reflects the current implementation rather than aspirational structure.

Notes

• Documentation-only change — no code behavior is modified.
• Addresses all items in the issue: overview, folder descriptions, architecture diagram, data flow, key modules, and dependencies.