Feat : Track focus sessions with XP and badges

[singhanurag0317-bit]

Summary

• persist completed Focus Mode sessions locally and associate completed sessions with the selected due-soon task
• convert focused study minutes into XP, levels, day streaks, and milestone badges in a new progress panel
• display focused time alongside each focus task with responsive styling for smaller screens

Scope

This implements the integrated study timer and gamified dashboard portion of the proposal. The AI study companion and chat features remain separate follow-up work.

Testing

• npm.cmd test
• node --check server.js
• node --experimental-default-type=module --check js/app.js
• git diff --check
• verified that the local server responds with the new Focus Mode progress markup

Addresses part of #753