feat: replace admin auth with cookie-based profile picker

Remove all authentication (login, sessions, bcrypt, itsdangerous) since
the app runs on a private homelab LAN. Replace with a profile picker
landing page and cookie-based profile selection (1-year expiry).

- Add Alembic migration to drop password_hash/is_admin columns
- Delete auth service, auth routes, login template, and auth tests
- Rewrite app/utils/auth.py with NoProfileSelectedError and
  require_active_profile dependency
- Add profile creation flow (GET/POST /profiles/create)
- Rewrite home page as profile picker with card layout
- Update all route files to use profile dependency instead of admin auth
- Remove bcrypt and itsdangerous from requirements
- Remove admin_username/admin_password from config
- Update all tests for new profile-based access model

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/partials/nav.html

@@ -1,7 +1,5 @@
-{% set admin = request.state.admin %}
 {% set profiles = request.state.profiles %}
 {% set active_profile = request.state.active_profile %}
-{% if admin %}
 <li>
     <details class="dropdown">
         <summary>
@@ -26,13 +24,10 @@
         </ul>
     </details>
 </li>
+<li><a href="/">Home</a></li>
 <li><a href="/workouts">Workouts</a></li>
 <li><a href="/schedule">Schedule</a></li>
 <li><a href="/dashboard">Dashboard</a></li>
 <li><a href="/history">History</a></li>
 <li><a href="/exercises">Exercises</a></li>
 <li><a href="/profiles">Profiles</a></li>
-<li><a href="/logout">Logout</a></li>
-{% else %}
-<li><a href="/login">Login</a></li>
-{% endif %}