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>

lib/theme/theme.dart

@@ -197,7 +197,11 @@ class ChainTheme {
   static ThemeData light() {
     final scheme = ColorScheme(
       brightness: Brightness.light,
-      primary: ChainColors.accentDeep,
+      // Sky-700, not sky-500: white button/label text on the lighter
+      // accent only reaches 2.8:1 — WCAG AA needs 4.5:1 (a11y_test.dart
+      // pins this). The dark theme keeps the lighter accent because it
+      // pairs it with near-black text.
+      primary: ChainColors.accentMuted,
       onPrimary: ChainColors.lightSurface,
       primaryContainer: const Color(0xFFE0F2FE),
       onPrimaryContainer: ChainColors.accentMuted,
@@ -205,7 +209,7 @@ class ChainTheme {
       onSecondary: ChainColors.lightSurface,
       secondaryContainer: ChainColors.lightSurfaceHigh,
       onSecondaryContainer: ChainColors.lightText,
-      tertiary: ChainColors.accentDeep,
+      tertiary: ChainColors.accentMuted,
       onTertiary: ChainColors.lightSurface,
       tertiaryContainer: const Color(0xFFE0F2FE),
       onTertiaryContainer: ChainColors.accentMuted,
@@ -261,6 +265,13 @@ class ChainTheme {
         ),
         margin: EdgeInsets.zero,
       ),
+      // Match FABs to the filled-button accent: Material 3 defaults
+      // them to primaryContainer, which reads as a washed-out tonal
+      // surface next to the sky-700 CTAs in the light theme.
+      floatingActionButtonTheme: FloatingActionButtonThemeData(
+        backgroundColor: scheme.primary,
+        foregroundColor: scheme.onPrimary,
+      ),
       filledButtonTheme: FilledButtonThemeData(
         style: FilledButton.styleFrom(
           backgroundColor: scheme.primary,