feat(sessions): implement device auto-detection and active session management UI

Description:

Implements automated device detection on login and active session management UI for user profiles, as part of issue #312.

When a user logs in, the app now automatically captures their device info (OS, browser, CPU, RAM) using browser APIs and sends it to the backend. A new Active Sessions dropdown is added to the profile page via a settings icon next to the logout button, allowing users to view all active login sessions and remotely sign out of other devices.

Changes Made:

• Added device info auto-detection on login using ua-parser-js (OS, browser, CPU cores, RAM, internet speed)
• Sends device_info payload to backend on login request
• Added ActiveSessions component showing all active login sessions
• Each session card shows OS, browser, IP address, last active time
• Added "This device" badge for current session
• Added "Sign out" button for individual session termination
• Added "Sign out of all other devices" button
• Dropdown closes on outside click or ✕ button
• Fixed mobile responsive layout for icons and dropdown
• Removed Active Sessions from bottom of profile page

Type of Change:

• Bug Fix
• Feature Development
• Documentation

Testing:

• Tested on desktop
• Tested on mobile
• Tested in different browsers

Screenshots (if applicable):

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Checklist:

• Code follows project style guidelines
• Self-review completed
• No console errors
• TypeScript types are correct

Depends On: Backend MR in corpus-server-app must be merged first. Backend implements the sessions table and API endpoints: GET /api/v1/sessions/ DELETE /api/v1/sessions/ DELETE /api/v1/sessions/{session_id}