feat: replace admin auth with cookie-based profile picker

Remove all authentication (login, sessions, bcrypt, itsdangerous) since
the app runs on a private homelab LAN. Replace with a profile picker
landing page and cookie-based profile selection (1-year expiry).

- Add Alembic migration to drop password_hash/is_admin columns
- Delete auth service, auth routes, login template, and auth tests
- Rewrite app/utils/auth.py with NoProfileSelectedError and
  require_active_profile dependency
- Add profile creation flow (GET/POST /profiles/create)
- Rewrite home page as profile picker with card layout
- Update all route files to use profile dependency instead of admin auth
- Remove bcrypt and itsdangerous from requirements
- Remove admin_username/admin_password from config
- Update all tests for new profile-based access model

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/pages/home.html

@@ -8,5 +8,33 @@
     <p>Your open-source workout tracker</p>
 </hgroup>
 
-<p>Welcome to SneakySwole. Get started by logging in.</p>
+{% if profiles %}
+<h2>Select Profile</h2>
+<div class="grid">
+    {% for profile in profiles %}
+    <article>
+        <header>
+            <strong>{{ profile.display_name }}</strong>
+        </header>
+        {% if profile.height or profile.weight %}
+        <p>
+            {% if profile.height %}Height: {{ profile.height }}{% endif %}
+            {% if profile.height and profile.weight %} &middot; {% endif %}
+            {% if profile.weight %}Weight: {{ profile.weight }}{% endif %}
+        </p>
+        {% endif %}
+        <footer>
+            <form method="POST" action="/profiles/switch">
+                <input type="hidden" name="profile_id" value="{{ profile.id }}">
+                <button type="submit" class="contrast">Select</button>
+            </form>
+        </footer>
+    </article>
+    {% endfor %}
+</div>
+{% else %}
+<p>No profiles yet. Create one to get started.</p>
+{% endif %}
+
+<a href="/profiles/create" role="button" class="secondary">Create New Profile</a>
 {% endblock %}