Implement chat scroll positioning without intermediate jumps

* scrollToIndex fails for unrendered messages; fallback using estimated message heights causes inaccurate scrolling (wrong message shown). * Opening chat with unread messages currently renders list first (often at bottom), then scrolls to last read — causes visible jump. * Re-entering chat should restore previous scroll position without rendering at bottom first. Affected features: Search messages → scroll to result Reply preview tap → scroll to replied message Unread messages → show last read position + unread header Back navigation → restore exact previous scroll position Requirement: Implement a single solution that scrolls directly to the target position on initial render — no intermediate render at bottom, no visible jump. Works for both: Last read message (with unread indicator) Previously saved scroll position