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