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/l10n/app_en.arb

@@ -573,6 +573,7 @@
   "auditFilterFlow": "flow",
   "auditFilterStep": "step",
   "auditFilterModule": "module",
+  "auditFilterTooltip": "Filter events",
   "auditClearLogTooltip": "Clear log (local/dev channels only)",
   "auditClearedToast": "Cleared {n} events on channel \"{channel}\". Chain reseeded with marker.",
   "@auditClearedToast": {
@@ -1220,7 +1221,7 @@
   "doctorDaemonFilesSection": "Daemon files",
   "doctorDaemonControlSection": "Daemon control",
   "doctorChainIntact": "Hash chain intact",
-  "doctorChainIntactDetail": "{n} events · prev_event_sha256 verified end-to-end",
+  "doctorChainIntactDetail": "{n} {n, plural, =1{event} other{events}} · prev_event_sha256 verified end-to-end",
   "@doctorChainIntactDetail": {
     "placeholders": {
       "n": {