fix: preserve per-session scroll position on session switch (#1083)

* fix: restore scroll position when switching chat sessions

When switching between chat sessions, scroll position now restores to
where the user left off instead of always jumping to the bottom.

- Save pixel-level scrollPosition (scrollTop/scrollHeight/clientHeight)
  in viewport store on every scroll event
- Add restoreSavedScrollPosition to timeline controller for ratio-based
  restoration (handles content size changes between visits)
- Suppress intermediate scroll events during session transition with an
  explicit flag, cleared deterministically after restore completes
- Cancel in-flight animations/follow-loops on session switch
- Preserve scrollPosition when session-ui-store rebuilds SessionMemoryState

* fix: keep streaming sessions pinned on restore

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/sync/session-ui-store.ts

@@ -819,10 +819,11 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       if (!memState || !memState.lastUserMessageAt) {
         const newMemState = new Map(viewportState.sessionMemoryState)
         newMemState.set(currentSessionId, {
-          viewportAnchor: memState?.viewportAnchor ?? 0,
-          isStreaming: memState?.isStreaming ?? false,
+          viewportAnchor: 0,
+          isStreaming: false,
           lastAccessedAt: Date.now(),
-          backgroundMessageCount: memState?.backgroundMessageCount ?? 0,
+          backgroundMessageCount: 0,
+          ...memState,
           lastUserMessageAt: Date.now(),
         })
         useViewportStore.setState({ sessionMemoryState: newMemState })