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/auth/src/commonMain/kotlin/at/mocode/frontend/core/auth/data/AuthApiClient.kt

@@ -3,7 +3,6 @@ package at.mocode.frontend.core.auth.data
 import at.mocode.shared.core.AppConstants
 import io.ktor.client.*
 import io.ktor.client.call.*
-import io.ktor.client.request.*
 import io.ktor.client.request.forms.*
 import io.ktor.http.*
 import kotlinx.serialization.Serializable

frontend/core/auth/src/commonMain/kotlin/at/mocode/frontend/core/auth/data/AuthTokenManager.kt

@@ -286,7 +286,7 @@ class AuthTokenManager {
    */
   private fun extractPermissionsFromJson(jsonString: String): List<String>? {
     return try {
-      // Simple regex to find permissions array
+      // Simple regex to find a permissions array
       val permissionsRegex = """"permissions":\s*\[(.*?)\]""".toRegex()
       val match = permissionsRegex.find(jsonString)