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/tests/test_widgets_tasks.py

@@ -76,9 +76,9 @@ def _capture_build_tasks_title(monkeypatch):
     seen_titles = []
     real_build_tasks = widgets.tasks._build_tasks
 
-    def spy(tasks, target_w, target_h, palette_rgb=None, title="Tasks"):
+    def spy(tasks, target_w, target_h, palette_rgb=None, title="Tasks", font_scale=1.0):
         seen_titles.append(title)
-        return real_build_tasks(tasks, target_w, target_h, palette_rgb, title)
+        return real_build_tasks(tasks, target_w, target_h, palette_rgb, title, font_scale=font_scale)
 
     monkeypatch.setattr(widgets.tasks, "_build_tasks", spy)
     return seen_titles