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_card.html

@@ -0,0 +1,28 @@
+<article>
+    <header>
+        <hgroup>
+            <h3>{{ exercise.name }}</h3>
+            <p>{{ exercise.muscle_group }} | {{ exercise.sets }} sets | Tempo: {{ exercise.tempo }}</p>
+        </hgroup>
+    </header>
+
+    {% if program %}
+    <div class="grid">
+        <div>
+            <small>Week 1</small>
+            <p>{{ program.wk1_reps }} reps @ {{ program.wk1_weight }}</p>
+        </div>
+        <div>
+            <small>Week 4</small>
+            <p>{{ program.wk4_reps }} reps @ {{ program.wk4_weight }}</p>
+        </div>
+    </div>
+    {% endif %}
+
+    <details>
+        <summary>Form Cues</summary>
+        <p>{{ exercise.form_cues }}</p>
+    </details>
+
+    <!-- Phase 4 adds: inline logging form here -->
+</article>

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 %}

app/templates/partials/flash_message.html

@@ -0,0 +1,6 @@
+{% if flash_success %}
+<div class="flash-success" role="status">{{ flash_success }}</div>
+{% endif %}
+{% if flash_error %}
+<div class="flash-error" role="alert">{{ flash_error }}</div>
+{% endif %}

app/templates/partials/nav.html

@@ -0,0 +1,35 @@
+{% set admin = request.state.admin %}
+{% set profiles = request.state.profiles %}
+{% set active_profile = request.state.active_profile %}
+{% if admin %}
+<li>
+    <details class="dropdown">
+        <summary>
+            {% if active_profile %}
+                {{ active_profile.display_name }}
+            {% else %}
+                Select Profile
+            {% endif %}
+        </summary>
+        <ul dir="rtl">
+            {% for profile in profiles %}
+            <li>
+                <form method="POST" action="/profiles/switch" style="margin:0;">
+                    <input type="hidden" name="profile_id" value="{{ profile.id }}">
+                    <button type="submit" class="outline secondary"
+                            style="width:100%; text-align:left; border:none;">
+                        {{ profile.display_name }}
+                    </button>
+                </form>
+            </li>
+            {% endfor %}
+        </ul>
+    </details>
+</li>
+<li><a href="/workouts">Workouts</a></li>
+<li><a href="/exercises">Exercises</a></li>
+<li><a href="/profiles">Profiles</a></li>
+<li><a href="/logout">Logout</a></li>
+{% else %}
+<li><a href="/login">Login</a></li>
+{% endif %}

app/templates/partials/profile_card.html

@@ -0,0 +1,12 @@
+<article>
+    <header>
+        <hgroup>
+            <h3>{{ profile.display_name }}</h3>
+            <p>{{ profile.height }} | {{ profile.weight }}</p>
+        </hgroup>
+    </header>
+    <p>{{ profile.goals or "No goals set" }}</p>
+    <footer>
+        <a href="/profiles/{{ profile.id }}/edit" role="button" class="outline">Edit</a>
+    </footer>
+</article>

app/templates/partials/warmup_list.html

@@ -0,0 +1,23 @@
+<table>
+    <thead>
+        <tr>
+            <th>Exercise</th>
+            <th>Type</th>
+            <th>Reps</th>
+        </tr>
+    </thead>
+    <tbody>
+        {% for warmup in warmups %}
+        <tr>
+            <td>
+                <details>
+                    <summary>{{ warmup.name }}</summary>
+                    <p>{{ warmup.form_cues }}</p>
+                </details>
+            </td>
+            <td>{{ warmup.type }}</td>
+            <td>{{ warmup.reps }}</td>
+        </tr>
+        {% endfor %}
+    </tbody>
+</table>