Add whiteboard frame mode (Nextcloud Whiteboard / Excalidraw over WebDAV)

New third mode alongside photos/calendar: fetches a .whiteboard file
over plain WebDAV (Basic auth -- generic, not Nextcloud-specific) and
renders it via a small Node.js sidecar using Excalidraw's own real
export code (@excalidraw/utils + @resvg/resvg-js, no headless browser),
since a .whiteboard file turns out to be Excalidraw scene JSON, not an
image. The sidecar runs as a second process inside this same container
(Dockerfile installs Node, start.sh backgrounds it before exec'ing
uvicorn) rather than a separate docker-compose service -- lightweight,
stateless, reachable only at 127.0.0.1 from the Python process, nothing
worth independently scaling.

The rendered PNG is treated exactly like a photo from there on --
composed/quantized through the existing image_pipeline (letterboxed,
never cropped) rather than a second parallel rendering pipeline.

WebDAV credentials support the common "it's actually the same Nextcloud
account as my CalDAV" case (an explicit opt-in checkbox, not silently
inferred) while still working with any WebDAV server generically.
Frame-level source (URL + owning account) follows the same owner-
controls-their-own-data permission split as calendar sources and the
week view's task list: only the account owner can point a frame at it,
anyone linked can clear it.

Honest limitation: this environment has no Node.js/npm, so
render-service/ is written carefully against each library's documented
API (verified via the npm registry, including transitive dependency
licenses after the CalDAV/AGPL surprise earlier this session) but has
never actually been executed. First real docker build is the first
true test -- see render-service/README.md.

server/app/static/settings.js

@@ -3,6 +3,19 @@
 // the frame's already-saved Immich creds) -- so this only works after
 // the CalDAV URL/username/password have been saved once.
 
+// Hides the dedicated WebDAV username/password fields while "reuse my
+// CalDAV creds" is checked -- they'd be ignored server-side anyway (see
+// routers/common.py's webdav_creds_for), no reason to leave them visibly
+// editable and implying they still do something.
+const reuseCaldavCreds = document.getElementById('webdav_reuse_caldav_creds');
+if (reuseCaldavCreds) {
+  const updateWebdavFieldVisibility = () => {
+    document.getElementById('webdav-creds-fields').style.display = reuseCaldavCreds.checked ? 'none' : '';
+  };
+  reuseCaldavCreds.addEventListener('change', updateWebdavFieldVisibility);
+  updateWebdavFieldVisibility();
+}
+
 const discoverBtn = document.getElementById('caldav-discover');
 if (discoverBtn) {
   discoverBtn.addEventListener('click', async () => {