fix: WCAG AA color compliance (Redesign Phase A)

- Accent colors: #3478F6 → #2563EB (5.17:1), dark mode → #60A5FA
- Semantic colors: success #15803D, warning #B45309, danger #DC2626
- Text tertiary: #737370 (4.76:1), dark #A3A3A0
- Module calendar dark: #A78BFA (5.28:1)
- Greeting gradient: #1D4ED8 → #2563EB (both ≥ 4.5:1)
- Theme-color harmonized to #2563EB across meta/manifest/tokens
- --text-2xs: 0.625rem → 0.6875rem (11px)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/manifest.json

@@ -4,8 +4,8 @@
   "description": "Selbstgehosteter Familienplaner für Kalender, Aufgaben, Einkauf und mehr.",
   "start_url": "/",
   "display": "standalone",
-  "background_color": "#F5F5F7",
-  "theme_color": "#007AFF",
+  "background_color": "#F5F4F1",
+  "theme_color": "#2563EB",
   "orientation": "portrait-primary",
   "lang": "de",
   "icons": [