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_base.html

@@ -29,6 +29,7 @@
         <span class="settings-nav-heading">You</span>
         <a class="{% if active_section == 'account' %}selected{% endif %}" href="{% url 'settings-account' %}">Account</a>
         <a class="{% if active_section == 'api-keys' %}selected{% endif %}" href="{% url 'settings-api-keys' %}">API Keys</a>
+        <a class="{% if active_section == 'labels' %}selected{% endif %}" href="{% url 'settings-labels' %}">Labels</a>
         <a class="{% if active_section == 'storage' %}selected{% endif %}" href="{% url 'settings-storage' %}">Storage</a>
         <a class="{% if active_section == 'demo-data' %}selected{% endif %}" href="{% url 'settings-demo-data' %}">Demo Data</a>
       </div>