refactor(auth, design-system): remove unused methods and annotations for cleanup

- Deleted obsolete methods such as `exchangeAuthorizationCode` and `logout` from the Auth module.
- Removed unused browser PKCE utilities and associated constants.
- Annotated unused components across the Design System with `@Suppress("unused")` for clarity.
- Simplified `LoginViewModel` to handle logout by clearing UI state directly.

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/AppScaffold.kt

@@ -7,6 +7,7 @@ import androidx.compose.material3.Scaffold
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 
+@Suppress("unused")
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
 fun AppScaffold(

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/LoadingIndicator.kt

@@ -51,6 +51,7 @@ fun LoadingIndicator(
   }
 }
 
+@Suppress("unused")
 @Composable
 fun FullScreenLoading(
   message: String = "Loading...",
@@ -67,6 +68,7 @@ fun FullScreenLoading(
   }
 }
 
+@Suppress("unused")
 @Composable
 fun InlineLoading(
   message: String? = null,
@@ -84,6 +86,7 @@ fun InlineLoading(
   }
 }
 
+@Suppress("unused")
 @Composable
 fun LinearLoadingIndicator(
   modifier: Modifier = Modifier,

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/MeldestelleButton.kt

@@ -87,6 +87,7 @@ private fun ButtonContent(
   }
 }
 
+@Suppress("unused")
 @Composable
 fun PrimaryButton(
   text: String,
@@ -105,6 +106,7 @@ fun PrimaryButton(
   fullWidth = fullWidth
 )
 
+@Suppress("unused")
 @Composable
 fun SecondaryButton(
   text: String,

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/MeldestelleTextField.kt

@@ -92,6 +92,7 @@ fun MeldestelleTextField(
   }
 }
 
+@Suppress("unused")
 @Composable
 fun MeldestellePasswordField(
   value: String,
@@ -136,6 +137,7 @@ fun MeldestellePasswordField(
   )
 }
 
+@Suppress("unused")
 @Composable
 fun MeldestelleEmailField(
   value: String,
@@ -169,6 +171,7 @@ fun MeldestelleEmailField(
 /**
  * Form validation utilities
  */
+@Suppress("unused")
 object FormValidation {
   fun validateEmail(email: String): String? {
     return when {

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/theme/AppTheme.kt

@@ -35,6 +35,7 @@ private val DarkColorScheme = darkColorScheme(
   onSurface = Color(0xFFE0E0E0)
 )
 
+@Suppress("unused")
 @Composable
 fun AppTheme(
   darkTheme: Boolean = false, // For now, we'll default to light theme