fix: action button on Reader now toggles auto-scroll (not food assistant)

Bug: when action tab (value=3) was tapped, selectedTab was already 3
by the time handleActionTap ran. The check 'if selectedTab == 2'
always failed, falling through to food assistant.

Fix: use onChange(of: selectedTab) oldValue to capture which tab the
user was on BEFORE tapping the action button. Pass that to
handleActionTap(from:). If from Reader (2), toggle auto-scroll.
If from Home/Fitness, open food assistant.

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

ios/Platform/Platform/ContentView.swift

@@ -30,6 +30,7 @@ struct MainTabView: View {
     @State private var readerVM = ReaderViewModel()
     @State private var isAutoScrolling = false
     @State private var scrollSpeed: Double = 1.0
+    @State private var previousTab = 0
 
     private var showReader: Bool {
         auth.currentUser?.id != 4
@@ -64,11 +65,7 @@ struct MainTabView: View {
                 // Home/Fitness: quick add food (+)
                 // Reader: play/pause auto-scroll
                 Tab(value: 3, role: .search) {
-                    // This view shows briefly when tapped — immediately redirect
                     Color.clear
-                        .onAppear {
-                            handleActionTap()
-                        }
                 } label: {
                     Label("Action", systemImage: actionIcon)
                 }
@@ -146,23 +143,28 @@ struct MainTabView: View {
             renderer.attachToWindow()
             await readerVM.loadInitial()
         }
-        .onChange(of: selectedTab) { _, newTab in
+        .onChange(of: selectedTab) { oldTab, newTab in
+            if newTab == 3 {
+                // Action tab tapped — handle based on previous tab
+                handleActionTap(from: oldTab)
+            } else {
+                previousTab = newTab
                 if newTab != 2 { isAutoScrolling = false }
             }
         }
+    }
 
-    private func handleActionTap() {
-        if selectedTab == 2 {
-            // Reader: toggle auto-scroll, stay on Reader
+    private func handleActionTap(from sourceTab: Int) {
+        if sourceTab == 2 {
+            // Reader: toggle auto-scroll, return to Reader
             withAnimation(.spring(duration: 0.3)) {
                 isAutoScrolling.toggle()
             }
-            selectedTab = 2 // stay on Reader (don't switch to the "action" tab)
+            selectedTab = 2
         } else {
             // Home/Fitness: open food assistant, return to previous tab
-            let returnTab = selectedTab
             showAssistant = true
-            selectedTab = returnTab
+            selectedTab = sourceTab
         }
     }