feat: Light Mode mit System-Erkennung und konfigurierbarem Erscheinungsbild.

Stellt hell/dunkel für Ocean, Material und Cupertino bereit, migriert die Kern-UI auf CSS-Variablen und ergänzt die Einstellungen inkl. i18n und Select-Kontrast.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/i18n/locales/en.json

@@ -286,6 +286,11 @@
       "theme_ocean": "Ocean (Glassmorphism)",
       "theme_material": "Material (Android)",
       "theme_cupertino": "Cupertino (iOS)",
+      "color_scheme_title": "Appearance",
+      "color_scheme_label": "Light or dark mode (default: follow system)",
+      "color_scheme_auto": "Auto (System)",
+      "color_scheme_light": "Light",
+      "color_scheme_dark": "Dark",
       "share_title": "Share Logbook (Read-Only)",
       "share_desc": "Enable this to generate a public, read-only link. Anyone with the link can view your travels, yacht profile, and crew members. Decryption keys are never transmitted to the server (they stay in the hash part of the URL).",
       "share_enable": "Enable Public Link",