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/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>