feat(ui): ModeBanner replaces DemoBanner + no silent mock-fallback

Two related fixes so the app never silently presents itself as
a demo when it is in fact running against a real hub:

  - The orange 'Demo-Daten · KEINE Rechtsberatung' banner was
    hard-coded into every data screen, regardless of whether
    the active repository was the in-memory MockRepository or
    the live HubRepository. Replaced with ModeBanner, which
    picks one of three voices by repository.mode:

      demo      orange T4 — 'Demo-Daten · KEINE Rechtsberatung'
      hub-live  petrol     — 'Live aus Hub · <endpoint>'
      hub-down  warm-warn  — 'Hub nicht erreichbar — leere Liste'

  - main.dart's _pickRepository used to fall back to
    MockRepository whenever HubRepository.connect() came back
    unhealthy. That silently downgraded a live deployment to
    demo data, which is exactly what the user is trying to avoid.
    Removed: when useHub=true, the HubRepository is returned
    regardless. Its list() returns [] on hub-down, and the
    'hub-down' banner explains why the screens are empty.
    Mock data only shows when the user explicitly turns useHub
    off in the Hub-Reiter.

Also: legend arrow icons now point in the actual axis direction
(→ for Schaden, ↑ for Nutzen) instead of the bidirectional ↔/↕,
which suggested the axes ran both ways. Mini-chips and the
expanded legend rows are consistent.

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

app/lib/pages/heatmap_page.dart

@@ -3,7 +3,7 @@ import 'package:flutter/material.dart';
 import '../data/models.dart';
 import '../data/repository.dart';
 import '../theme/reclaim_tokens.dart';
-import '../widgets/demo_banner.dart';
+import '../widgets/mode_banner.dart';
 import '../widgets/heatmap_grid.dart';
 import '../widgets/reclaim_card.dart';
 import '../widgets/tier_badge.dart';
@@ -32,7 +32,7 @@ class _HeatmapPageState extends State<HeatmapPage> {
     final t = Theme.of(context).textTheme;
     return Column(
       children: [
-        const DemoBanner(),
+        ModeBanner(repository: widget.repository),
         Expanded(
           child: Padding(
             padding: const EdgeInsets.all(ReclaimSpace.xl),
@@ -190,8 +190,8 @@ class _LegendCardState extends State<_LegendCard> {
                       spacing: ReclaimSpace.lg,
                       runSpacing: ReclaimSpace.xs,
                       children: [
-                        _MiniLabel(symbol: '↔', text: 'Schaden'),
-                        _MiniLabel(symbol: '↕', text: 'Nutzen'),
+                        _MiniLabel(symbol: '→', text: 'Schaden'),
+                        _MiniLabel(symbol: '↑', text: 'Nutzen'),
                         _MiniLabel(
                             symbol: '●', text: 'Betroffenheit'),
                         _MiniLabel(
@@ -219,7 +219,7 @@ class _LegendCardState extends State<_LegendCard> {
                               CrossAxisAlignment.start,
                           children: [
                             _LegendRow(
-                              symbol: '↔',
+                              symbol: '→',
                               axis: 'Abszisse (X)',
                               label: 'Schaden',
                               description:
@@ -231,7 +231,7 @@ class _LegendCardState extends State<_LegendCard> {
                             ),
                             SizedBox(height: ReclaimSpace.sm),
                             _LegendRow(
-                              symbol: '↕',
+                              symbol: '↑',
                               axis: 'Ordinate (Y)',
                               label: 'Nutzen',
                               description:

app/lib/pages/norm_detail_page.dart

@@ -3,7 +3,7 @@ import 'package:flutter/material.dart';
 import '../data/models.dart';
 import '../data/repository.dart';
 import '../theme/reclaim_tokens.dart';
-import '../widgets/demo_banner.dart';
+import '../widgets/mode_banner.dart';
 import '../widgets/evidence_sidebar.dart';
 import '../widgets/norm_text_card.dart';
 import '../widgets/reclaim_card.dart';
@@ -29,7 +29,7 @@ class NormDetailPage extends StatelessWidget {
       ),
       body: Column(
         children: [
-          const DemoBanner(),
+          ModeBanner(repository: repository),
           Expanded(
             child: SingleChildScrollView(
               padding: const EdgeInsets.all(ReclaimSpace.xl),

app/lib/pages/norms_list_page.dart

@@ -3,7 +3,7 @@ import 'package:flutter/material.dart';
 import '../data/models.dart';
 import '../data/repository.dart';
 import '../theme/reclaim_tokens.dart';
-import '../widgets/demo_banner.dart';
+import '../widgets/mode_banner.dart';
 import '../widgets/reclaim_card.dart';
 import '../widgets/tier_badge.dart';
 import 'norm_detail_page.dart';
@@ -18,7 +18,7 @@ class NormsListPage extends StatelessWidget {
     final t = Theme.of(context).textTheme;
     return Column(
       children: [
-        const DemoBanner(),
+        ModeBanner(repository: repository),
         Expanded(
           child: SingleChildScrollView(
             padding: const EdgeInsets.all(ReclaimSpace.xl),