Implement real API clients and update types

Replace mock data in geocoding, HAFAS, and bike routing clients with
actual implementations using fetch and appropriate interfaces. Update
typescript definitions to match the new data structures, including
Journey, Station, and BikeRoute. Add vitest and related testing
dependencies, replacing the placeholder test script with actual tests
for the new client logic. Refactor calendar and countdown utilities to
use the new types and remove unused files.

Implement real API clients and update types

Replace mock data in HafasClient, GeocodingClient, and
BikeRoutingClient with actual fetch implementations. Update
types to match API responses and add Vitest tests.

eslint.config.mjs

@@ -13,6 +13,15 @@ const eslintConfig = defineConfig([
     "build/**",
     "next-env.d.ts",
   ]),
+  {
+    rules: {
+      // Allow _-prefixed parameters and variables to signal intentionally unused.
+      "@typescript-eslint/no-unused-vars": [
+        "warn",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+    },
+  },
 ]);
 
 export default eslintConfig;