The desktop terminal page sometimes grew an outer scrollbar because the
terminal instance/wrapper/container chain lacked height constraints.
Without min/max-height enforcement, xterm.js's internal viewport could
expand its parent flex/grid track past the available space.
- Add overflow: hidden to .terminal-page.
- Add max-height: 100% and overflow: hidden to .terminal-instance.
- Add max-height: 100% to .terminal-wrapper.
- Add min-height: 0 to .terminal-container.
- Constrain .xterm-viewport to max-height/width 100% so it fills but
never exceeds its container.
Quality gates: npm run typecheck, npm run lint, npm test -- --run (87 passed).
Refs: openspec/changes/fix-terminal-container-overflow
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