fix(approvals,l10n,theme): usertest findings — localized prompts, no-data confirm, .chain log path, honest wording, AA contrast

- approvals: empty/legacy hub prompts render the localized fallback;
  approving without show: data asks for conscious confirmation first
- chain_log: write to ~/.chain/logs/studio-errors.log (was .fai),
  one-time best-effort migration of the legacy file + rotation sibling
- l10n: 'manipulationssicher' -> 'manipulationserkennend', neutral
  WORM-1 blurb, doctor pill 'Integritätskette v1', federation hint
  says the CA authenticates the first connect (DE+EN)
- theme: muted text token now >=4.5:1 on canvas, cards and elevated
  dark surfaces (was 3.7:1 on cards)

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

lib/theme/tokens.dart

@@ -27,7 +27,9 @@ class ChainColors {
   static const surface = Color(0xFF18181B); // cards
   static const surfaceHigh = Color(0xFF27272A); // elevated
   static const border = Color(0xFF3F3F46); // 1px outlines
-  static const muted = Color(0xFF71717A); // de-emphasised text
+  // De-emphasised text. WCAG AA (≥4.5:1) against canvas, cards AND
+  // elevated surfaces — 0xFF71717A only reached 3.7:1 on cards.
+  static const muted = Color(0xFF8E8E97);
   static const text = Color(0xFFE4E4E7); // body
   static const textStrong = Color(0xFFFAFAFA); // headings