refactor(web-ui): stabilize flow chat scroll with viewport state machine

[GCWing]

Summary

• Replace legacy Virtuoso scroll adapters and geometry helpers with a unified FlowViewport state machine and controller for more stable chat scrolling.
• Wire session/navigation header context through the workspace shell and simplify session scene navigation.
• Remove ScrollToBottomButton and related auto-scroll hooks superseded by the new viewport system.

Test plan

• pnpm run type-check:web
• Open a long Agentic OS session and verify streaming auto-follow behavior
• Scroll up during streaming and confirm follow mode disengages until user returns to bottom
• Switch sessions and confirm scroll position resets correctly
• Verify welcome panel and session header title updates on navigation