feat(01-02): design token system, i18n framework, and theme provider setup

- OKLCH design tokens with yellow #ffed00 primary and dark gray-blue dark mode
- next-intl cookie-based locale with DE/EN message files
- next-themes provider with system/light/dark support
- Sidebar and header layout dimension tokens
- Root layout with ThemeProvider and NextIntlClientProvider wrappers

apps/web/src/messages/de.json

@@ -0,0 +1,40 @@
+{
+  "common": {
+    "appName": "Tessera",
+    "loading": "Laden...",
+    "save": "Speichern",
+    "cancel": "Abbrechen"
+  },
+  "header": {
+    "search": "Suchen...",
+    "userMenu": "Benutzermenu",
+    "breadcrumb": {
+      "home": "Startseite"
+    }
+  },
+  "sidebar": {
+    "dashboard": "Dashboard",
+    "marketplace": "Marktplatz",
+    "settings": "Einstellungen",
+    "collapse": "Einklappen",
+    "expand": "Ausklappen",
+    "categories": {
+      "label": "Kategorien",
+      "empty": "Keine Module"
+    }
+  },
+  "dashboard": {
+    "title": "Dashboard",
+    "empty": "Keine Widgets aktiv",
+    "addWidget": "Widget hinzufuegen"
+  },
+  "theme": {
+    "light": "Hell",
+    "dark": "Dunkel",
+    "system": "System"
+  },
+  "locale": {
+    "de": "Deutsch",
+    "en": "English"
+  }
+}

apps/web/src/messages/en.json

@@ -0,0 +1,40 @@
+{
+  "common": {
+    "appName": "Tessera",
+    "loading": "Loading...",
+    "save": "Save",
+    "cancel": "Cancel"
+  },
+  "header": {
+    "search": "Search...",
+    "userMenu": "User menu",
+    "breadcrumb": {
+      "home": "Home"
+    }
+  },
+  "sidebar": {
+    "dashboard": "Dashboard",
+    "marketplace": "Marketplace",
+    "settings": "Settings",
+    "collapse": "Collapse",
+    "expand": "Expand",
+    "categories": {
+      "label": "Categories",
+      "empty": "No modules"
+    }
+  },
+  "dashboard": {
+    "title": "Dashboard",
+    "empty": "No active widgets",
+    "addWidget": "Add widget"
+  },
+  "theme": {
+    "light": "Light",
+    "dark": "Dark",
+    "system": "System"
+  },
+  "locale": {
+    "de": "Deutsch",
+    "en": "English"
+  }
+}