feat(brand): BrandWordmark renders ∆I in petrol, lifts the logo

Adds a dedicated BrandWordmark widget that splits 'Recl∆Im'
into three TextSpans and tints the middle ∆I in the petrol
signal colour. The fai_web universe uses this same accent rule
for the F∆I and Ch∆In glyphs — Recl∆Im now follows it, so
the brand stays visually coherent across every F∆I surface.

Where it lands:

  - Landing page: large DeltaMark (96 px) and BrandWordmark
    (56 px) sit on a single row, the wordmark to the right of
    the mark. The two together carry the brand identity that
    a viewer sees first when opening the app.
  - NavigationRail leading: the muted 'Recl∆Im' Text line is
    replaced with the petrol-accented BrandWordmark (18 px),
    keeping the vendor tag 'F∆I' as a small muted line under it.

widget_test.dart adapts: the literal 'Recl∆Im'-find no longer
matches because the wordmark is now three text spans, so the
test checks for BrandWordmark by type — equivalent assertion,
robust to glyph-level splits.

Signed-off-by: flemming-it <sf@flemming.it>

app/lib/pages/shell_page.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../data/repository.dart';
 import '../theme/reclaim_tokens.dart';
+import '../widgets/brand_wordmark.dart';
 import '../widgets/delta_mark.dart';
 import 'about_page.dart';
 import 'heatmap_page.dart';
@@ -51,14 +52,7 @@ class _ShellPageState extends State<ShellPage> {
                 children: [
                   const DeltaMark(size: 36),
                   const SizedBox(height: ReclaimSpace.sm),
-                  Text(
-                    'Recl∆Im',
-                    style: t.labelLarge?.copyWith(
-                      letterSpacing: 1.5,
-                      color: ReclaimColors.signal,
-                      fontWeight: FontWeight.w600,
-                    ),
-                  ),
+                  const BrandWordmark(size: 18),
                   Text(
                     'F∆I',
                     style: t.labelSmall?.copyWith(