added XP system

[Mulesneha]

Related Issue

Closes #273

Summary

Implemented a persistent XP (experience points) system that automatically tracks and rewards users for study activity including time spent studying, task completions, topic completions, and notes completion.

Changes Made

• Added xp_system.js — Express router with SQLite-backed XP ledger and session tables
• Added xp_client.js — Browser-side XP client with XPClient, XPWidget, integrateXpWithStore, and attachXpToTimer utilities
• Added xp_integration.js — Drop-in wiring module; call initXP(store) to activate the full system
• Patched store.toggleTaskStatus to automatically award XP on task completion
• Wired Focus Mode timer to track study time and flush XP on session end or page close
• Added floating XP badge widget (level, XP total, progress bar) that mounts into the existing UI
• Integrated XP award toasts with the existing Toast utility

Testing

• Verified XP is awarded correctly for each action type (task done, topic done, notes done, study time)
• Confirmed anti-abuse guards work: task/note XP awarded only once per ID; topic XP once per day; study time capped at 120 min per call and deduplicated via session token
• Tested pagehide/beforeunload beacon flush to ensure study time is not lost on tab close
• Confirmed level computation thresholds (Lv1→2: 100 XP, Lv2→3: 200 XP, etc.)
• Tested API endpoints manually: GET /api/xp, GET /api/xp/ledger, POST /api/xp/task-done, POST /api/xp/topic-done, POST /api/xp/notes-done, POST /api/xp/study-time, POST /api/xp/award
• Verified XP widget re-renders correctly on XP change events

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)