Keep proposal card metadata fixed during expansion

[Cyber-preacher]

Why

Expanding a proposal card replaced its two-column header with a single-column layout, moving the chamber pill, stage pill, date, and chevron.

Changes

• preserve the proposal card header grid in expanded state
• anchor the metadata grid so a growing summary cannot recenter it
• add a regression contract preventing expanded-state layout overrides

Validation

• 241 unit tests passed
• typecheck passed
• production build passed
• Prettier and diff checks passed
• Playwright coordinate check confirmed zero x/y/width movement for both pills, date, and chevron after expansion