fix(todo): Today's tab centres its day in the calendar

The day stood on the calendar's bottom edge: a line box's place in the
space below the calendar's line was the font's to choose. It is placed by
its baseline now.

Whats-New: Today's tab shows its day centred in the calendar.
Whats-New-De: Der Heute-Tab zeigt den Tag mittig im Kalender.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/src/tabs/today_icon.dart

@@ -8,6 +8,12 @@ import 'package:material_ui/material_ui.dart';
 /// blank calendar with today's day of the month in it, as Lucide's
 /// `calendar-1` holds a 1, drawn anew at midnight.
 class const TodayIcon({super.key}) extends StatefulWidget {
+  /// The day's size, in the calendar's 24 units.
+  static const _fontSize = 9.6;
+
+  /// Noto Sans' digits' height, of the font's size.
+  static const _digitHeight = 0.714;
+
   @override
   State<TodayIcon> createState() => _TodayIconState();
 }
@@ -49,22 +55,27 @@ class _TodayIconState() extends State<TodayIcon> {
         alignment: Alignment.center,
         children: [
           const Icon(LucideIcons.calendar),
-          // Lucide's calendar: its box from 4 to 22 of 24 units, the line
-          // under its rings at 10; the day centred in what is below it.
-          Positioned(
-            top: size * 10 / 24,
-            bottom: size * 2 / 24,
-            left: 0,
-            right: 0,
-            child: Center(
-              child: Text(
-                '${clock.now().day}',
-                textScaler: TextScaler.noScaling,
-                style: TextStyle(
-                  color: icon.color,
-                  fontSize: size * 0.4,
-                  height: 1,
-                  fontWeight: FontWeight.w700,
+          // Lucide's calendar: inside its strokes from 11 to 21 of 24 units.
+          // Placed by its baseline, a line box's place in it being the font's
+          // to choose (the day stood on the frame, 5 Nov 2026).
+          Positioned.fill(
+            child: Align(
+              alignment: Alignment.topCenter,
+              child: Baseline(
+                baseline:
+                    size *
+                    (16 + TodayIcon._digitHeight * TodayIcon._fontSize / 2) /
+                    24,
+                baselineType: TextBaseline.alphabetic,
+                child: Text(
+                  '${clock.now().day}',
+                  textScaler: TextScaler.noScaling,
+                  style: TextStyle(
+                    color: icon.color,
+                    fontSize: size * TodayIcon._fontSize / 24,
+                    height: 1,
+                    fontWeight: FontWeight.w700,
+                  ),
                 ),
               ),
             ),