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/shells/meldestelle-portal/src/jsMain/kotlin/Config.kt

@@ -0,0 +1,26 @@
+import kotlinx.browser.window
+import kotlinx.coroutines.await
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+
+@Serializable
+data class AppConfig(
+    val apiBaseUrl: String
+)
+
+suspend fun loadAppConfig(): AppConfig {
+    return try {
+        // Fetch config.json generated by Caddy templates
+        val response = window.fetch("/config.json").await()
+        if (!response.ok) {
+            console.warn("[Config] Failed to load config.json, falling back to defaults")
+            return AppConfig(apiBaseUrl = window.location.origin)
+        }
+        val text = response.text().await()
+        Json.decodeFromString(AppConfig.serializer(), text)
+    } catch (e: dynamic) {
+        console.error("[Config] Error loading config:", e)
+        // Fallback for local development if file is missing
+        AppConfig(apiBaseUrl = "http://localhost:8081")
+    }
+}