fix(frontend): align chat navigator and clarify org assets

[nessie993]

Summary

• Make ChatTurnNavigator markers span the chat viewport so the final marker clearly represents the bottom.
• Keep the navigator hit area in the left gutter while constraining previews to the chat pane width.
• Update the org assets copy and render final interaction content before activity details.

Verification

• yarn test src/components/session/ChatTurnNavigator.test.ts src/components/session/ChatTurnNavigator.component.test.tsx
• yarn vitest run --exclude 'lib/**' — 323 passed, 1 skipped
• yarn tsc --pretty false
• yarn build

The live preview tab was reachable at http://localhost:8080, but its browser snapshot/evaluation timed out, so no browser visual verification is claimed.