AUTH-004: OAuth login — Sign in with GitHub

## Overview
Add GitHub OAuth as a login option. Users can authenticate with their GitHub account without creating a separate password.

## Acceptance Criteria
- [ ] 'Sign in with GitHub' button on the login page
- [ ] GitHub OAuth flow redirects back to TechMart and creates/links an account
- [ ] Existing accounts with matching email are automatically linked
- [ ] OAuth session uses the same token format as email/password auth

## Depends On
- #37 (AUTH-001 — session management)

Start at the login page entry point and review the session-management work in #37 before implementing the GitHub OAuth flow. Done means the button, redirect, account creation or email-based linking, and shared token format all satisfy the acceptance criteria.

Written by the indexing model from the issue text.