refactor(ui, navigation): implement platform-specific routing and redesign components

- Added platform detection logic `currentPlatform()` in `PlatformType.js.kt`.
- Introduced platform-based behavior for LandingScreen, Dashboard, and Login flow.
- Replaced Row with FlowRow in PingScreen to improve button layout.
- Updated Meldestelle Dashboard with platform-specific headers and authentication checks.
- Adjusted AppHeader to accept `isAuthenticated` and `username` parameters.

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

frontend/core/domain/src/commonMain/kotlin/at/mocode/frontend/core/domain/PlatformType.kt

@@ -0,0 +1,8 @@
+package at.mocode.frontend.core.domain
+
+enum class PlatformType {
+  WEB,
+  DESKTOP
+}
+
+expect fun currentPlatform(): PlatformType

frontend/core/domain/src/jsMain/kotlin/at/mocode/frontend/core/domain/PlatformType.js.kt

@@ -0,0 +1,3 @@
+package at.mocode.frontend.core.domain
+
+actual fun currentPlatform(): PlatformType = PlatformType.WEB

frontend/core/domain/src/jvmMain/kotlin/at/mocode/frontend/core/domain/PlatformType.jvm.kt

@@ -0,0 +1,3 @@
+package at.mocode.frontend.core.domain
+
+actual fun currentPlatform(): PlatformType = PlatformType.DESKTOP

frontend/core/domain/src/wasmJsMain/kotlin/at/mocode/frontend/core/domain/PlatformType.wasmJs.kt

@@ -0,0 +1,3 @@
+package at.mocode.frontend.core.domain
+
+actual fun currentPlatform(): PlatformType = PlatformType.WEB