fix: Save to Collection button not responding without user interaction

The Save to Collection button on the identification screen would remain
disabled after plant identification completed, requiring the user to tap
a prediction row before it would enable. This was caused by an @Observable
+ @State tracking issue where computed properties in SwiftUI view modifiers
don't always trigger re-renders when the underlying observable changes.

Replaced the empty .onChange workaround with a local @State property
(saveEnabled) that is explicitly updated when selectedPrediction or
saveState changes, ensuring the button state always reflects the current
ViewModel state.

Fixes #1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PlantGuide/Presentation/Scenes/Identification/IdentificationView.swift

@@ -19,6 +19,11 @@ struct IdentificationView: View {
     /// Tracks whether we've announced results to avoid duplicate announcements
     @State private var hasAnnouncedResults = false
 
+    /// Local state to reliably drive the save button's enabled/disabled state.
+    /// Works around an @Observable + @State tracking issue where computed
+    /// properties in view modifiers don't always trigger re-renders.
+    @State private var saveEnabled = false
+
     // MARK: - Scaled Metrics for Dynamic Type
 
     @ScaledMetric(relativeTo: .body) private var closeIconSize: CGFloat = 16
@@ -76,8 +81,10 @@ struct IdentificationView: View {
                 announceStateChange(from: oldValue, to: newValue)
             }
             .onChange(of: viewModel.selectedPrediction?.id) { _, _ in
-                // Force view update when selection changes
-                // This ensures SwiftUI tracks @Observable property changes correctly
+                saveEnabled = viewModel.canSaveToCollection
+            }
+            .onChange(of: viewModel.saveState) { _, _ in
+                saveEnabled = viewModel.canSaveToCollection
             }
             .accessibilityIdentifier(AccessibilityIdentifiers.Identification.identificationView)
             .alert("Plant Saved!", isPresented: .init(
@@ -406,12 +413,12 @@ struct IdentificationView: View {
                     .padding(.vertical, 14)
                     .background(
                         RoundedRectangle(cornerRadius: 12)
-                            .fill(viewModel.canSaveToCollection ? Color.accentColor : Color.gray)
+                            .fill(saveEnabled ? Color.accentColor : Color.gray)
                     )
                 }
-                .disabled(!viewModel.canSaveToCollection)
+                .disabled(!saveEnabled)
                 .accessibilityLabel(viewModel.saveState == .saving ? "Saving plant" : "Save to Collection")
-                .accessibilityHint(viewModel.canSaveToCollection ? "Saves the selected plant to your collection" : "Select a plant first")
+                .accessibilityHint(saveEnabled ? "Saves the selected plant to your collection" : "Select a plant first")
                 .accessibilityIdentifier(AccessibilityIdentifiers.Identification.saveToCollectionButton)
             }
             .padding(.horizontal, 20)