v0.7.1: in-app password change, custom labels, always-on folders

- Settings -> Account: change your own password (current + new), session
  preserved via update_session_auth_hash.
- Custom labels: per-user palette in Settings -> Labels; apply/remove from a
  message's ... menu (menu stays open, checkmark = applied). Labels are unified
  with tags; chip colours sync to the global TagColor. New Label model +
  migration 0017; POST /a/{id}/label (session, owner-scoped, palette-validated).
- Sidebar Keep/Deleted folders are now always shown; count badge only when
  non-empty. Updates the stale 'hide when empty' note in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/settings_account.html

@@ -10,4 +10,27 @@
     revocable on its own. You have {{ key_count }} key{{ key_count|pluralize }}.
   </p>
   <p><a href="{% url 'settings-api-keys' %}">Manage API keys &rarr;</a></p>
+
+  <h3>Change password</h3>
+  <p>Enter your current password, then choose a new one.</p>
+  <form method="post" class="password-change">
+    {% csrf_token %}
+    {% if password_form.non_field_errors %}
+      <ul class="form-errors">
+        {% for error in password_form.non_field_errors %}<li>{{ error }}</li>{% endfor %}
+      </ul>
+    {% endif %}
+    {% for field in password_form %}
+      <label class="form-field">
+        <span>{{ field.label }}</span>
+        {{ field }}
+        {% if field.errors %}
+          <ul class="form-errors">
+            {% for error in field.errors %}<li>{{ error }}</li>{% endfor %}
+          </ul>
+        {% endif %}
+      </label>
+    {% endfor %}
+    <button type="submit">Change password</button>
+  </form>
 {% endblock %}