refactor(theme): centralize colors and typography in AppColors and AppTypography

- Replaced hardcoded color values in `LightColorScheme` and `DarkColorScheme` with `AppColors`.
- Updated `AppTypography` references in `AppMaterialTypography`.
- Upgraded Kotlin, Compose Multiplatform, and Ktor to latest versions in `libs.versions.toml`.
- Added `uiToolingPreview` dependency in `meldestelle-portal` build script.
- Refactored config loading to include `RequestInit`.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

frontend/shells/meldestelle-portal/build.gradle.kts

@@ -83,6 +83,7 @@ kotlin {
       implementation(compose.ui)
       implementation(compose.components.resources)
       implementation(compose.materialIconsExtended)
+      implementation(compose.components.uiToolingPreview)
 
       // Bundles
       implementation(libs.bundles.kmp.common)        // Coroutines, Serialization, DateTime