Settings: add dedicated DAV/Pushover tabs, fix CalDAV/CardDAV bugs

- Add admin DAV tab (rename from CalDAV/CardDAV) and Pushover tab
  - Add per-user Pushover tab (User Key only; App Token stays admin-managed)
  - Remove system-wide CalDAV/CardDAV fallback — per-user config only
  - Rewrite contacts_tool.py using httpx directly (caldav 2.x dropped AddressBook)
  - Fix CardDAV REPORT/PROPFIND using SOGo URL pattern
  - Fix CalDAV/CardDAV test endpoints (POST method, URL scheme normalization)
  - Fix Show Password button — API now returns actual credential values
  - Convert Credentials tab to generic key-value store; dedicated keys
    (CalDAV, Pushover, trusted_proxy) excluded via _DEDICATED_CRED_KEYS

server/web/templates/chats.html

@@ -128,6 +128,7 @@ function renderChats(data) {
            data-id="${c.id}" onclick="localStorage.setItem('current_session_id',this.dataset.id)">Open</a>
         <button class="btn btn-ghost btn-small"
                 data-id="${c.id}" onclick="openRenameModal(this.dataset.id)">Rename</button>
+        <a class="btn btn-ghost btn-small" href="/api/conversations/${c.id}/export" download>Export</a>
         <button class="btn btn-ghost btn-small" style="color:var(--danger,#dc3c3c)"
                 data-id="${c.id}" onclick="deleteSingle(this.dataset.id)">Delete</button>
       </div>