feat: add Phase 3 Workout UI — auth, profiles, workout viewer, exercise browser

Build the core user-facing experience with admin login (bcrypt + signed
session cookies), profile switcher, workout day viewer with warmups and
exercise cards, and HTMX-powered exercise browser with search/filter.

- AuthService with bcrypt password verification and itsdangerous session tokens
- Auth dependency redirects to /login (303) for unauthenticated requests
- NavContextMiddleware injects admin/profiles/active_profile into all templates
- Profile management (list, switch, edit) with cookie-based active profile
- Workout day viewer shows warmups + exercises + per-user programming targets
- Exercise browser with HTMX filter dropdowns (no page reloads)
- Flash message partial for success/error feedback
- 12 new tests (66 total passing)

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

app/templates/base.html

@@ -27,14 +27,16 @@
             </ul>
             <ul>
                 {% block nav_items %}
-                <!-- Phase 3 adds: profile switcher, login/logout -->
+                {% include "partials/nav.html" ignore missing %}
                 {% endblock %}
             </ul>
         </nav>
     </header>
 
     <main class="container">
-        {% block flash %}{% endblock %}
+        {% block flash %}
+        {% include "partials/flash_message.html" ignore missing %}
+        {% endblock %}
         {% block content %}{% endblock %}
     </main>