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/templates/settings.html

@@ -71,6 +71,32 @@
         you're linked to from that frame's Calendar tab, so a frame only
         shows calendars people have actually chosen to share with it.</p>
 
+      <h2 class="card-title" style="margin-top: 24px;">Whiteboard (WebDAV)</h2>
+      <p class="sub">Credentials for whiteboard frame mode -- fetching a
+        specific file (e.g. a Nextcloud Whiteboard board) over WebDAV.
+        Any WebDAV server works, not just Nextcloud.</p>
+      <div class="checkbox-row">
+        <input type="checkbox" id="webdav_reuse_caldav_creds" name="webdav_reuse_caldav_creds"
+               value="true" {% if user.webdav_reuse_caldav_creds %}checked{% endif %}>
+        <label for="webdav_reuse_caldav_creds" style="margin: 0; font-weight: normal;">
+          Reuse my CalDAV username/password above (only works if it's the
+          same account -- e.g. Nextcloud's CalDAV and its regular file
+          storage share one login)</label>
+      </div>
+      <div id="webdav-creds-fields">
+        <label>WebDAV username
+          <input type="text" id="webdav_username" name="webdav_username" autocomplete="off"
+                 value="{{ user.webdav_username }}">
+        </label>
+        <label>WebDAV password
+          <input type="password" id="webdav_password" name="webdav_password" autocomplete="off"
+                 placeholder="{% if user.webdav_password %}(unchanged -- enter a new password to replace){% else %}app password or account password{% endif %}">
+        </label>
+      </div>
+      <p class="sub" style="margin-top: 8px;">Doesn't show up anywhere by
+        itself -- point a specific frame's Whiteboard tab at a file URL
+        using this account.</p>
+
       <h2 class="card-title" style="margin-top: 24px;">Change password</h2>
       <label>Current password
         <input type="password" name="current_password" autocomplete="current-password">