Add a per-widget text-size picker for calendar/tasks legibility

Calendar and tasks pack the most body text at the smallest default
sizes, so those two gear-icon dialogs get a "Text size" card (Normal/
Large/X-Large) alongside the existing Border card -- a new Widget-level
font_scale column with its own POST .../font-scale endpoint, same
Widget-property-not-config-field shape as border_style. Threaded through
every classic (calendar_render.py) and modern (html_render.py/
calendar_html_render.py) size calc via one shared panel_style.
scaled_size() so row heights/max_rows already derived from font size
re-fit around the bigger text automatically.

server/app/templates/_widget_dialog_calendar.html

@@ -135,6 +135,8 @@
 
 {% include "_widget_border_fields.html" %}
 
+{% include "_widget_font_scale_fields.html" %}
+
 {% include "_widget_button_fields.html" %}
 
 <section class="card" style="margin-top: 20px;">

server/app/templates/_widget_dialog_tasks.html

@@ -67,6 +67,8 @@
 
 {% include "_widget_border_fields.html" %}
 
+{% include "_widget_font_scale_fields.html" %}
+
 {% include "_widget_button_fields.html" %}
 
 <section class="card" style="margin-top: 20px;">

server/app/templates/_widget_font_scale_fields.html

@@ -0,0 +1,15 @@
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Text size</h2>
+  <p class="sub">Scales this widget's own text up for easier reading on
+    the panel -- rows/columns re-fit around the bigger text automatically.</p>
+  <form id="font-scale-config-form">
+    <label>Size
+      <select id="widget_font_scale">
+        {% for value, label in font_scale_choices %}
+          <option value="{{ value }}" {% if widget.font_scale == value %}selected{% endif %}>{{ label }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <button type="submit">Save</button>
+  </form>
+</section>

server/app/templates/frame_layout.html

@@ -77,6 +77,7 @@
   <script src="/static/widget_dialog_weather.js"></script>
   <script src="/static/widget_dialog_battery.js"></script>
   <script src="/static/widget_dialog_border.js"></script>
+  <script src="/static/widget_dialog_font_scale.js"></script>
   <script src="/static/widget_dialog_button_actions.js"></script>
   <script src="/static/frame_layout.js"></script>
   <script src="/static/saved_layouts.js"></script>