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/login.html

@@ -0,0 +1,27 @@
+{% extends "base.html" %}
+
+{% block title %}SneakySwole — Login{% endblock %}
+
+{% block content %}
+<article>
+    <header>
+        <h2>Admin Login</h2>
+    </header>
+
+    {% if error %}
+    <div class="flash-error" role="alert">{{ error }}</div>
+    {% endif %}
+
+    <form method="POST" action="/login">
+        <label for="username">Username</label>
+        <input type="text" id="username" name="username"
+               placeholder="Enter username" required autofocus>
+
+        <label for="password">Password</label>
+        <input type="password" id="password" name="password"
+               placeholder="Enter password" required>
+
+        <button type="submit">Login</button>
+    </form>
+</article>
+{% endblock %}