[detroitpro]

Summary

• On wide layouts, the thread terminal docks under the right-panel surfaces and shares their column width, so it stays visible while switching Files, browser, diff, and other tabs.
• Closing either the panel or the terminal leaves the other in place; with only the terminal open the column is terminal-only. Narrow sheet layout keeps the drawer under chat.
• Adds RightSidebarStack plus focused layout helpers/tests; updates appearance docs for the new placement.

Test plan

• vp test run on RightSidebarStack logic, RightPanelTabs, ThreadTerminalDrawer, PreviewPanelShell
• vp run --filter @t3tools/web typecheck
• Wide layout: open Files + terminal, switch to browser/diff — terminal stays mounted underneath
• Close right panel with terminal open — column becomes terminal-only at panel width
• Close terminal with panel open — surfaces fill the column
• Narrow viewport (≤980px): terminal still under chat; right panel remains a sheet
• Maximize right panel with terminal docked — stack fills; restore returns chat

Closes #7