Integrate Bootstrap 5 and Bootstrap Icons into UI

- Add Bootstrap 5.3.3 + Icons via CDN to base.html
- Replace 315-line hand-written CSS with 35-line brand overrides
- Update all 13 templates with Bootstrap utility classes:
  - Responsive navbar with mobile hamburger menu
  - Consistent card-based layout for forms and profile
  - Proper button alignment with d-flex and d-grid utilities
  - List groups for data display (profile info)
  - Professional alerts and icons
- No backend changes, no build toolchain needed
- Responsive design works on mobile/tablet/desktop

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

requirements.txt

@@ -2,8 +2,10 @@ fastapi==0.115.0
 uvicorn[standard]==0.30.0
 sqlalchemy==2.0.35
 pymysql==1.1.1
+cryptography>=41.0.0
 jinja2==3.1.4
 python-multipart==0.0.12
 passlib[bcrypt]==1.7.4
 bcrypt==4.2.0
 pydantic-settings==2.5.2
+itsdangerous==2.1.2