feat(auth, ui): add Keycloak login flow and enhance LoginScreen UI

- Introduced `onKeycloakLogin` callback in LoginScreen for Keycloak-based authentication.
- Updated Login UI with "Login with Keycloak" button and loading indicator for OIDC flow.
- Implemented dividers and spacing for improved visual structure.

- Refactored LandingScreen into a separate file for better modularity and maintenance.
- Reintroduced LandingScreen features, hero section, and footer with enhanced composables.

- Added `composeHotReload` plugin and `uiTooling` dependency for development improvements.

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

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

@@ -14,6 +14,7 @@ plugins {
   alias(libs.plugins.composeCompiler)
   alias(libs.plugins.composeMultiplatform)
   alias(libs.plugins.kotlinSerialization)
+  alias(libs.plugins.composeHotReload)
 }
 
 kotlin {
@@ -93,6 +94,7 @@ kotlin {
     jvmMain.dependencies {
       implementation(compose.desktop.currentOs)
       implementation(libs.kotlinx.coroutines.swing)
+      implementation(compose.uiTooling)
       implementation(libs.koin.core)
       implementation(project(":frontend:features:nennung-feature"))
     }