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/exercise_browser.html

@@ -0,0 +1,38 @@
+{% extends "base.html" %}
+
+{% block title %}Exercise Library — SneakySwole{% endblock %}
+
+{% block content %}
+<hgroup>
+    <h1>Exercise Library</h1>
+    <p>Browse all exercises with form cues and programming details.</p>
+</hgroup>
+
+<!-- HTMX-powered filters -->
+<div class="grid">
+    <select name="workout_day"
+            hx-get="/exercises/search"
+            hx-target="#exercise-results"
+            hx-include="[name='muscle_group']">
+        <option value="">All Days</option>
+        {% for day in workout_days %}
+        <option value="{{ day.name }}">{{ day.name }}</option>
+        {% endfor %}
+    </select>
+
+    <select name="muscle_group"
+            hx-get="/exercises/search"
+            hx-target="#exercise-results"
+            hx-include="[name='workout_day']">
+        <option value="">All Muscle Groups</option>
+        {% for mg in muscle_groups %}
+        <option value="{{ mg }}">{{ mg }}</option>
+        {% endfor %}
+    </select>
+</div>
+
+<!-- Exercise results (replaced by HTMX on filter change) -->
+<div id="exercise-results">
+    {% include "partials/exercise_list.html" %}
+</div>
+{% endblock %}