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_widget_border.py

@@ -37,7 +37,8 @@ def test_set_border_persists(client, db_session):
     assert resp.status_code == 200, resp.text
     assert resp.json() == {
         "id": widget_id, "widget_type": "photos", "x": 0, "y": 0, "w": 8, "h": 5, "sort_order": 0,
-        "border_style": "dashed", "border_thickness": 5, "border_color_index": 3, "locked": False,
+        "border_style": "dashed", "border_thickness": 5, "border_color_index": 3, "font_scale": 1.0,
+        "locked": False,
     }
     widget = db_session.get(Widget, widget_id)
     assert (widget.border_style, widget.border_thickness, widget.border_color_index) == ("dashed", 5, 3)