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/partials/exercise_list.html

@@ -0,0 +1,16 @@
+{% for exercise in exercises %}
+<article>
+    <header>
+        <hgroup>
+            <h3>{{ exercise.name }}</h3>
+            <p>{{ exercise.muscle_group }} | {{ exercise.workout_day }} Day | {{ exercise.sets }} sets | Tempo: {{ exercise.tempo }}</p>
+        </hgroup>
+    </header>
+    <details>
+        <summary>Form Cues</summary>
+        <p>{{ exercise.form_cues }}</p>
+    </details>
+</article>
+{% else %}
+<p>No exercises found matching your filters.</p>
+{% endfor %}