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/pages/workout_days.html

@@ -0,0 +1,22 @@
+{% extends "base.html" %}
+
+{% block title %}Workout Days — SneakySwole{% endblock %}
+
+{% block content %}
+<h1>Workout Days</h1>
+
+<div class="grid">
+    {% for day in days %}
+    <article>
+        <header>
+            <h3>Day {{ day.day_number }}: {{ day.name }}</h3>
+        </header>
+        <p>{{ day.description }}</p>
+        <footer>
+            <a href="/workouts/{{ day.name|lower|replace(' ', '-') }}"
+               role="button">View Workout</a>
+        </footer>
+    </article>
+    {% endfor %}
+</div>
+{% endblock %}