feat: add runtime configuration for Caddy-based SPA containerization

Introduced `config.json` runtime configuration fetch mechanism to support the "Build Once, Deploy Everywhere" pattern. Replaced NGINX with Caddy for SPA deployment, enabling SPA routing, security headers, and static asset management. Updated Gradle and Kotlin/JS build configurations to align with the new runtime environment. Enhanced Dockerfile and health checks for optimized CI/CD workflows and improved SPA delivery.

frontend/features/ping-feature/src/commonTest/kotlin/at/mocode/ping/feature/test/Fakes.kt

@@ -132,8 +132,8 @@ class TestPingApiClient : PingApi {
     return handleRequest(securePingResponse)
   }
 
-  override suspend fun syncPings(lastSyncTimestamp: Long): List<PingEvent> {
-    syncPingsCalledWith = lastSyncTimestamp
+  override suspend fun syncPings(since: Long): List<PingEvent> {
+    syncPingsCalledWith = since
     callCount++
 
     if (simulateDelay) {