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/frame_header.js

@@ -71,9 +71,12 @@
       });
       if (!resp.ok) throw new Error(await apiError(resp));
       previous = mode;
-      showStatus(true, mode === 'calendar' ? 'Switched to Calendar mode.' : 'Switched to Photos mode.');
+      var modeLabels = { photos: 'Photos', calendar: 'Calendar', whiteboard: 'Whiteboard' };
+      showStatus(true, `Switched to ${modeLabels[mode] || mode} mode.`);
       var calTab = document.querySelector('.tabs a[href$="/calendar"]');
       if (calTab) calTab.classList.toggle('tab-disabled', mode !== 'calendar');
+      var wbTab = document.querySelector('.tabs a[href$="/whiteboard"]');
+      if (wbTab) wbTab.classList.toggle('tab-disabled', mode !== 'whiteboard');
     } catch (e) {
       sel.value = previous;
       showStatus(false, e.message);