add student mode with streak tracking

[rashmisatvika]

Closes #221

What's changed

• Added two-faced identity system (General Mode + Student Mode)
• Student onboarding flow: name + daily study hour goal (3 step)
• Daily study session logging (hours + notes)
• Streak system: gains +1 on consecutive days, drops -1 per missed day
• Dashboard with streak ring, stats and recent sessions
• History screen with weekly bar chart and full session log
• Settings screen: edit name, goal, clear all data
• Full REST API backend (Express + SQLite) with session and habit endpoints
• Sand & Stone warm beige/terracotta theme applied throughout

New files added

• frontend/src/pages/student/page.jsx — Student Mode frontend
• backend/server.js — API server
• backend/package.json — backend dependencies

How to test

1. cd backend && npm install && npm run dev
2. cd frontend && npm run dev
3. Open app → select Student identity
4. Complete onboarding → log a session → check streak updates

Screenshots

UI includes:

• Identity selection screen
• 3 step onboarding
• Student dashboard with streak ring
• History screen with weekly chart
• Settings screen