Refactor: Apply padding on a per-screen basis

This change refactors the padding application from a global Scaffold modifier to a per-screen basis.

- The global `padding(innerPadding)` is removed from the `Surface` in `MainActivity`.
- `innerPadding` is now passed down to `AppNavigation` and then to individual screens (`MenuScreen`, `PhotoReasoningRoute`).
- `MenuScreen` applies the full `innerPadding` to maintain its original layout.
- `PhotoReasoningScreen` applies a custom padding, omitting the top padding to allow content to shift upwards.

Author: google-labs-jules[bot]

app/src/main/kotlin/com/google/ai/sample/MainActivity.kt

@@ -45,6 +45,7 @@ import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.PaddingValues
 import androidx.compose.foundation.layout.padding
 import androidx.compose.material3.Button
 import androidx.compose.material3.Card

app/src/main/kotlin/com/google/ai/sample/MenuScreen.kt

@@ -358,19 +358,19 @@ fun MenuScreen(
 @Composable
 fun MenuScreenPreview() {
     // Preview with trial not expired
-    MenuScreen(isTrialExpired = false, isPurchased = false)
+    MenuScreen(innerPadding = PaddingValues(), isTrialExpired = false, isPurchased = false)
 }
 
 @Preview(showSystemUi = true)
 @Composable
 fun MenuScreenPurchasedPreview() {
-    MenuScreen(isTrialExpired = false, isPurchased = true)
+    MenuScreen(innerPadding = PaddingValues(), isTrialExpired = false, isPurchased = true)
 }
 
 @Preview(showSystemUi = true)
 @Composable
 fun MenuScreenTrialExpiredPreview() {
     // Preview with trial expired
-    MenuScreen(isTrialExpired = true, isPurchased = false)
+    MenuScreen(innerPadding = PaddingValues(), isTrialExpired = true, isPurchased = false)
 }
 

app/src/main/kotlin/com/google/ai/sample/feature/multimodal/PhotoReasoningScreen.kt

@@ -12,6 +12,7 @@ import androidx.activity.result.PickVisualMediaRequest
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.PaddingValues
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxWidth
@@ -1075,6 +1076,7 @@ fun ErrorChatBubble(
 fun PhotoReasoningScreenPreviewWithContent() {
     MaterialTheme {
         PhotoReasoningScreen(
+            innerPadding = PaddingValues(),
             uiState = PhotoReasoningUiState.Success("This is a preview of the photo reasoning screen."),
             commandExecutionStatus = "Command executed: Take screenshot",
             detectedCommands = listOf(
@@ -1188,6 +1190,7 @@ val SystemMessageEntrySaver = Saver<SystemMessageEntry?, List<String?>>(
 fun PhotoReasoningScreenPreviewEmpty() {
     MaterialTheme {
         PhotoReasoningScreen(
+            innerPadding = PaddingValues(),
             isKeyboardOpen = false,
             onStopClicked = {},
             isInitialized = true