Live-update whiteboard/tasks source display; add firmware CI build check

Whiteboard and tasks-source save/clear used to just tell the user to
reload the page to see the change. Both endpoints always assign a
successful "set" to the calling user, so the new state is fully known
client-side already -- rewrite the "Currently using/showing ..." block
in place instead, no server round trip or reload needed.

Also adds .gitea/workflows/firmware-build-check.yml: builds both board
variants (devkit, xiao) on every push touching firmware/**, unlike
firmware-release-build.yml which only builds on a version.txt bump.
Verified both builds succeed locally against the actual ESP-IDF
toolchain before wiring this in.

server/app/templates/frame_calendar.html

@@ -163,13 +163,15 @@
           <label for="tasks_enabled">Show a task list</label>
         </div>
 
-        {% if tasks_source %}
-          <p class="sub" style="margin-top: 10px;">
-            Currently using <strong>{{ tasks_source.display_name }}</strong>'s
-            <strong>{{ tasks_source.label }}</strong> list.
-            <button type="button" class="btn-inline secondary" id="tasks-source-clear">Clear</button>
-          </p>
-        {% endif %}
+        <div id="tasks-current-source">
+          {% if tasks_source %}
+            <p class="sub" style="margin-top: 10px;">
+              Currently using <strong>{{ tasks_source.display_name }}</strong>'s
+              <strong>{{ tasks_source.label }}</strong> list.
+              <button type="button" class="btn-inline secondary" id="tasks-source-clear">Clear</button>
+            </p>
+          {% endif %}
+        </div>
 
         {% if viewer_task_calendars %}
           <p class="sub" style="margin-top: 10px;">Use one of your own CalDAV task lists:</p>

server/app/templates/frame_whiteboard.html

@@ -27,20 +27,22 @@
           scene) fetched over WebDAV -- credentials set up in
           <a href="/settings">Settings</a>.</p>
 
-        {% if whiteboard_source %}
-          <p class="sub" style="margin-top: 10px;">
-            Currently showing <strong>{{ whiteboard_source.url }}</strong>
-            using <strong>{{ whiteboard_source.display_name }}</strong>'s
-            WebDAV account.
-            <button type="button" class="btn-inline secondary" id="whiteboard-source-clear">Clear</button>
-          </p>
-        {% else %}
-          <p class="sub" style="margin-top: 10px;">No whiteboard configured yet.</p>
-        {% endif %}
+        <div id="whiteboard-current-source">
+          {% if whiteboard_source %}
+            <p class="sub" style="margin-top: 10px;">
+              Currently showing <strong>{{ whiteboard_source.url }}</strong>
+              using <strong>{{ whiteboard_source.display_name }}</strong>'s
+              WebDAV account.
+              <button type="button" class="btn-inline secondary" id="whiteboard-source-clear">Clear</button>
+            </p>
+          {% else %}
+            <p class="sub" style="margin-top: 10px;">No whiteboard configured yet.</p>
+          {% endif %}
+        </div>
 
         {% if viewer_has_webdav_creds %}
           <form id="whiteboard-source-form" style="margin-top: 16px;">
-            <label>{{ "Change to one of your own files" if whiteboard_source else "Use one of your own files" }}
+            <label><span id="whiteboard-source-form-label">{{ "Change to one of your own files" if whiteboard_source else "Use one of your own files" }}</span>
               <input type="text" id="whiteboard-url-input"
                      placeholder="https://cloud.example.com/remote.php/dav/files/you/Boards/family.whiteboard"
                      value="{{ whiteboard_source.url if whiteboard_source and whiteboard_source.user_id == user.id else '' }}">