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

Dockerfile

@@ -21,6 +21,9 @@ RUN apt-get update \
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
+# Install Playwright browser (Chromium) and its system dependencies
+RUN playwright install --with-deps chromium
+
 COPY server/ ./server/
 
 # Data directory for encrypted DB (mounted as volume in production)