fix(ui): WCAG light-theme accent, overflow-safe audit page, formal address

Accessibility/responsive audit pass with two new permanent test
gates (test/a11y_test.dart: WCAG text contrast + labeled tap
targets on every page in both themes; test/responsive_test.dart:
no layout overflow at 800/960/1280/1920 px). Findings fixed:

- Light theme primary/tertiary sky-500 → sky-700: white text on
  the lighter accent only reached 2.8:1 (welcome CTA, active
  sidebar label); sky-700 clears WCAG AA at ~5.9:1. Dark theme
  unchanged (already compliant). FABs now follow the same accent
  instead of Material 3's washed-out tonal default.
- Audit page: filter chips collapse into a checkmark popup menu
  below 900 px window width (app bar overflowed); the live-status
  bar's left text is now Expanded with ellipsis so the row can
  shrink, and the disconnected state's copyable error gets the
  full remaining width.
- German strings now use formal address consistently (~20 strings
  still used du-forms next to Sie-forms on welcome/setup), the
  audit event-type chip "Step" is "Schritt", and the doctor
  page's event count pluralises correctly in both languages.

flutter analyze clean, 64 tests green. Screenshot pass light+dark
via the guide-shots harness (verified parity, no overflows).

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/a11y_test.dart

@@ -0,0 +1,98 @@
+// Accessibility gate for every sidebar page, light AND dark:
+//
+//  * textContrastGuideline — WCAG AA text contrast (>= 4.5:1 for
+//    normal text, >= 3:1 for large text) on whatever the page
+//    renders without a hub (empty/offline states included; those
+//    are exactly the states a fresh operator sees first).
+//  * labeledTapTargetGuideline — every tappable target exposes a
+//    semantic label, so icon-only buttons must carry a tooltip or
+//    Semantics label. Screen-reader users get a name for every
+//    action.
+//
+// The Android/iOS tap-target SIZE guidelines are deliberately not
+// applied: Studio is a desktop app driven by pointer, and Flutter's
+// desktop defaults (e.g. 34px DropdownMenu items) fail the 48px
+// mobile rule by design.
+//
+// Navigation mirrors sidebar_test.dart: pump StudioApp directly,
+// tap the sidebar destination keys, pump a fixed duration (the app
+// holds long-lived timers, so pumpAndSettle never settles).
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:chain_studio/data/hub.dart';
+import 'package:chain_studio/main.dart';
+
+const _destinations = <String>[
+  'welcome',
+  'store',
+  'doctor',
+  'flows',
+  'audit',
+  'approvals',
+  'runs',
+  'federation',
+];
+
+void main() {
+  for (final mode in [ThemeModeValue.light, ThemeModeValue.dark]) {
+    testWidgets('all pages meet a11y guidelines — ${mode.name}',
+        (tester) async {
+      SharedPreferences.setMockInitialValues({});
+      // Layout correctness across sizes is responsive_test.dart's
+      // job; this suite audits colors and labels at a normal size.
+      tester.view.physicalSize = const Size(1280, 800);
+      tester.view.devicePixelRatio = 1.0;
+      addTearDown(tester.view.reset);
+      final handle = tester.ensureSemantics();
+      await tester.pumpWidget(
+        StudioApp(
+          initialThemeMode: mode,
+          initialLocale: const Locale('de'),
+        ),
+      );
+      await tester.pump(const Duration(milliseconds: 100));
+
+      // Collect violations across ALL pages before failing, so one
+      // regression does not mask the rest of the sweep.
+      final violations = <String>[];
+      for (final id in _destinations) {
+        await tester.tap(find.byKey(ValueKey('sidebar-item-$id')));
+        await tester.pump(const Duration(milliseconds: 400));
+        for (final (guideline, what) in [
+          (textContrastGuideline, 'text contrast'),
+          (labeledTapTargetGuideline, 'unlabeled tap targets'),
+        ]) {
+          final result = await guideline.evaluate(tester);
+          if (!result.passed) {
+            final reason = result.reason ?? '';
+            // Known evaluator false positive: the workspace switcher
+            // wraps its trigger in an OverlayPortal + Tooltip, which
+            // makes the contrast guideline lose the text render and
+            // measure the chip's own fill (#27272A) against the page
+            // canvas instead. The actual label text is onSurface on
+            // surfaceContainerHighest (>10:1). Skip exactly that node.
+            if (what == 'text contrast' &&
+                reason.contains('_OverlayPortalState') &&
+                reason.contains('Arbeitsbereich')) {
+              continue;
+            }
+            violations.add('page "$id" (${mode.name}) $what: $reason');
+          }
+        }
+      }
+      handle.dispose();
+      // Tear the app down explicitly and drain in-flight one-shot
+      // timers (status polls, tooltip delays) — the binding asserts
+      // !timersPending after the test body.
+      await tester.pumpWidget(const SizedBox.shrink());
+      await tester.pump(const Duration(minutes: 1));
+      expect(
+        violations,
+        isEmpty,
+        reason: 'a11y violations:\n${violations.join('\n\n')}',
+      );
+    });
+  }
+}