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/static/widget_dialog_calendar.js

@@ -199,6 +199,7 @@ function initCalendarDialog() {
   document.getElementById('calendar-preview-refresh').addEventListener('click', loadCalendarPreview);
   loadCalendarPreview();
   initBorderFields();
+  initFontScaleFields();
   initButtonActionFields();
 }
 

server/app/static/widget_dialog_font_scale.js

@@ -0,0 +1,32 @@
+// Shared "Text size" card (models.Widget.font_scale,
+// _widget_font_scale_fields.html) -- only present on the calendar/tasks
+// dialogs (see frame_pages.py's widget_dialog), same "one shared init
+// function" shape as initBorderFields, just not included on every
+// dialog since it isn't relevant to every widget type.
+
+function initFontScaleFields() {
+  const select = document.getElementById('widget_font_scale');
+  if (!select) return; // dialog fragment didn't render the font-scale card -- shouldn't happen
+
+  document.getElementById('font-scale-config-form').addEventListener('submit', async (e) => {
+    e.preventDefault();
+    try {
+      const resp = await fetch(`${window.FRAME_API}/font-scale`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ font_scale: Number(select.value) }),
+      });
+      if (!resp.ok) throw new Error(await apiError(resp));
+      showStatus(true, 'Text size saved.');
+      // Whichever dialog is actually open -- both loadCalendarPreview and
+      // loadTasksPreview are always defined (plain script tags, not
+      // module-scoped), so checking the function's existence isn't
+      // enough; check for the <img> it actually targets instead (calling
+      // the wrong one throws setting .src on a null element).
+      if (document.getElementById('calendar-preview')) loadCalendarPreview();
+      if (document.getElementById('tasks-preview')) loadTasksPreview();
+    } catch (err) {
+      showStatus(false, err.message);
+    }
+  });
+}

server/app/static/widget_dialog_tasks.js

@@ -90,6 +90,7 @@ function initTasksDialog() {
   document.getElementById('tasks-preview-refresh').addEventListener('click', loadTasksPreview);
   loadTasksPreview();
   initBorderFields();
+  initFontScaleFields();
   initButtonActionFields();
 }