Add a whiteboard file picker and force-refresh

File picker: an optional WebDAV browse root in Settings
(User.webdav_base_url) plus a plain-PROPFIND directory listing
(webdav_client.list_directory) power a "Browse..." panel on a frame's
Whiteboard tab, so a file can be clicked into rather than typing its
exact WebDAV URL. Manual URL entry still works unchanged either way.

Force-refresh: get_or_refresh_whiteboard takes a force flag that skips
the fetch throttle entirely; the Whiteboard tab's refresh button now
passes it, so clicking it always re-fetches and re-renders instead of
possibly just re-showing the same cached image from within the last
~20 minutes.

server/app/templates/frame_whiteboard.html

@@ -48,7 +48,18 @@
             <p class="sub" style="margin-top: 4px;">The direct WebDAV URL
               to the specific file -- in Nextcloud's Files app, this is
               the file's path under
-              <code>remote.php/dav/files/&lt;your-username&gt;/</code>.</p>
+              <code>remote.php/dav/files/&lt;your-username&gt;/</code>.
+              <button type="button" class="btn-inline secondary" id="whiteboard-browse-toggle">Browse...</button></p>
+
+            <div id="whiteboard-browser" style="display: none; margin-top: 8px; border: 1px solid var(--border-color, #ccc); border-radius: 6px; padding: 8px;">
+              <div style="display: flex; align-items: center; gap: 8px; margin-bottom: 6px;">
+                <button type="button" class="btn-inline secondary" id="whiteboard-browse-up" disabled>Up</button>
+                <span class="sub" id="whiteboard-browse-current" style="word-break: break-all;"></span>
+              </div>
+              <ul id="whiteboard-browse-list" style="list-style: none; margin: 0; padding: 0; max-height: 260px; overflow-y: auto;"></ul>
+              <p class="sub" id="whiteboard-browse-error" style="display: none; color: var(--error-color, #c00);"></p>
+            </div>
+
             <button type="submit">Save</button>
           </form>
         {% else %}
@@ -64,7 +75,7 @@
         <h2 class="card-title">Preview</h2>
         <p class="sub">How this frame's whiteboard currently renders.</p>
         <img class="preview-img" id="whiteboard-preview" alt="Whiteboard preview">
-        <button type="button" class="secondary" id="whiteboard-preview-refresh">Refresh preview</button>
+        <button type="button" class="secondary" id="whiteboard-preview-refresh">Refresh now</button>
       </section>
     </div>
   </div>