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/test/tabs/today_icon_test.dart

@@ -0,0 +1,36 @@
+import 'package:clock/clock.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_todo_flutter/src/tabs/today_icon.dart';
+import 'package:material_ui/material_ui.dart';
+
+void main() {
+  testWidgets("the day's digits stand centred below the calendar's line, "
+      "whatever the font's line box (the developer's review, 5 Nov 2026)", (
+    tester,
+  ) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IconTheme(
+          data: IconThemeData(size: 24),
+          child: Center(child: TodayIcon()),
+        ),
+      ),
+    );
+
+    final icon = tester.getTopLeft(find.byType(TodayIcon));
+    final day = find.text('${clock.now().day}');
+    final measured = TextPainter(
+      text: TextSpan(text: '5', style: tester.widget<Text>(day).style),
+      textDirection: TextDirection.ltr,
+    )..layout();
+    addTearDown(measured.dispose);
+    final baseline =
+        tester.getTopLeft(day).dy +
+        measured.computeDistanceToActualBaseline(TextBaseline.alphabetic) -
+        icon.dy;
+
+    // The digits, 0.714 of 9.6 units high, between the strokes at 11 and 21.
+    expect(baseline, moreOrLessEquals(19.43, epsilon: 0.01));
+  });
+}