Add a static image widget (PNG/JPEG/GIF/BMP/WEBP/TIFF/PDF upload)

A new widget type showing a single user-uploaded image with no live
upstream to poll -- decoded once at upload time (PDF's first page via
pypdfium2, BSD-3-Clause/Apache-2.0, no copyleft exposure) into plain RGB
PNG bytes, then composed per a crop/stretch/letterbox display mode like
the photos widget.

server/app/templates/_widget_dialog_static.html

@@ -0,0 +1,44 @@
+<h2 class="dialog-title">Static image widget</h2>
+
+<section class="card">
+  <h2 class="card-title">Image</h2>
+  <p class="sub">Upload a PNG, JPEG, GIF, BMP, WEBP, TIFF, or PDF (first
+    page only) -- it's decoded once on upload and shown as-is until you
+    upload something else.</p>
+  <p class="sub" id="static-current-file">
+    {% if static_cfg and static_cfg.original_filename %}
+      Currently showing: <strong>{{ static_cfg.original_filename }}</strong>
+    {% else %}
+      No image uploaded yet.
+    {% endif %}
+  </p>
+  <input type="file" id="static-file" accept="image/*,application/pdf">
+  <button type="button" class="secondary" id="static-upload" style="margin-top: 8px;">Upload</button>
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Settings</h2>
+  <form id="static-config-form">
+    <label>Display mode
+      <select id="display_mode">
+        {% for mode, label in display_mode_labels.items() %}
+          <option value="{{ mode }}" {% if static_cfg and static_cfg.display_mode == mode %}selected{% endif %}>{{ label }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <p class="sub" style="margin-top: 8px;">How the image's aspect ratio
+      is reconciled with this widget's box: <strong>Crop to fill</strong>
+      trims the excess; <strong>Stretch to fill</strong> fills the box
+      exactly without cropping (an image with a different aspect ratio
+      looks stretched); <strong>Shrink to fit</strong> shows the whole
+      image, letterboxed if needed.</p>
+    <button type="submit">Save</button>
+  </form>
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Preview</h2>
+  <p class="sub">How this widget currently renders.</p>
+  <img class="preview-img" id="static-preview" alt="Static image preview">
+  <button type="button" class="secondary" id="static-preview-refresh">Refresh now</button>
+</section>

server/app/templates/frame_layout.html

@@ -59,5 +59,6 @@
   <script src="/static/widget_dialog_calendar.js"></script>
   <script src="/static/widget_dialog_whiteboard.js"></script>
   <script src="/static/widget_dialog_tasks.js"></script>
+  <script src="/static/widget_dialog_static.js"></script>
   <script src="/static/frame_layout.js"></script>
 {% endblock %}