feat: persist scroll position per conversation tab

- Added scrollPosition to Conversation interface
- Save scroll position when switching away from a tab
- Restore exact scroll position when switching back
- Uses -1 to indicate auto-scroll mode (scroll to bottom)
- Prevents interference between scroll restore and auto-scroll

src/lib/stores/conversations.ts

@@ -21,6 +21,7 @@ export interface Conversation {
   grantedTools: Set<string>;
   pendingPermission: PermissionRequest | null;
   pendingQuestion: UserQuestionEvent | null;
+  scrollPosition: number;
   createdAt: Date;
   lastActivityAt: Date;
 }
@@ -55,6 +56,7 @@ function createConversationsStore() {
       grantedTools: new Set(),
       pendingPermission: null,
       pendingQuestion: null,
+      scrollPosition: -1, // -1 means "scroll to bottom" (auto-scroll)
       createdAt: new Date(),
       lastActivityAt: new Date(),
     };
@@ -106,6 +108,7 @@ function createConversationsStore() {
     ($conv) => $conv?.pendingPermission || null
   );
   const pendingQuestion = derived(activeConversation, ($conv) => $conv?.pendingQuestion || null);
+  const scrollPosition = derived(activeConversation, ($conv) => $conv?.scrollPosition ?? -1);
 
   return {
     // Expose derived stores for compatibility
@@ -118,6 +121,7 @@ function createConversationsStore() {
     isProcessing: { subscribe: isProcessing.subscribe },
     grantedTools: { subscribe: grantedTools.subscribe },
     pendingRetryMessage: { subscribe: pendingRetryMessage.subscribe },
+    scrollPosition: { subscribe: scrollPosition.subscribe },
 
     // New conversation-specific stores
     conversations: { subscribe: conversations.subscribe },
@@ -325,6 +329,22 @@ function createConversationsStore() {
       });
     },
 
+    saveScrollPosition: (id: string, position: number) => {
+      conversations.update((convs) => {
+        const conv = convs.get(id);
+        if (conv) {
+          conv.scrollPosition = position;
+        }
+        return convs;
+      });
+    },
+
+    getScrollPosition: (id: string): number => {
+      const convs = get(conversations);
+      const conv = convs.get(id);
+      return conv?.scrollPosition ?? -1;
+    },
+
     // Methods that operate on the active conversation
     setSessionId: (id: string | null) => {
       ensureInitialized();