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/panel_style.py

@@ -36,6 +36,28 @@ CONTENT_MARGIN = 20
 CARD_RADIUS = 12
 CHIP_RADIUS = 4
 
+# models.Widget.font_scale's allowed values -- a per-widget text-size
+# multiplier (the Layout dialog's "Text size" picker), same "reject
+# invalid, don't silently coerce" posture as border_style. Deliberately a
+# small fixed set (a <select>, not a raw slider) rather than an arbitrary
+# float: every _AGENDA_FONTS/_TASKS_FONTS-style tuple in calendar_render.py
+# and every proportional size in html_render.py/calendar_html_render.py
+# derives row heights/max_rows from the same scaled font size, so an
+# unbounded scale risks a layout that no longer fits its own box.
+FONT_SCALE_CHOICES = (1.0, 1.25, 1.5)
+
+
+def scaled_size(value: float, font_scale: float) -> int:
+    """Applies a widget's font_scale to a text-size value and rounds to
+    an int px, floored at 1 so an extreme scale can never zero out a
+    font. Shared by both the classic (PIL, calendar_render.py) and modern
+    (HTML/CSS, html_render.py/calendar_html_render.py) renderers so "make
+    this widget's text bigger" behaves identically regardless of
+    render_style -- every caller applies this immediately after its own
+    tier lookup/floor calc, so row heights/max_rows computed from the
+    result already account for the bigger text."""
+    return max(1, round(value * font_scale))
+
 # Index constants into DEFAULT_PALETTE_RGB/a frame's own Frame.
 # palette_rgb override -- same order as image_pipeline.PALETTE_LABELS.
 BLACK, WHITE, YELLOW, RED, BLUE, GREEN = range(6)