Add responsive and accessibility acceptance for capability surfaces

[sam2tom]

Outcome

Add narrow-screen, dark-mode, keyboard, and overflow acceptance coverage for the new Dashboard, Vaults, and Source Files surfaces.

Current behavior

The core workflows have component and functional coverage, while their cross-surface visual/accessibility acceptance is thinner than the established conversation views.

Acceptance criteria

• Exercise Dashboard, Vaults, and Source Files at a 390 px viewport.
• Verify no unintended horizontal page overflow or clipped primary actions.
• Exercise dark mode for readable status, error, and empty states.
• Verify keyboard-reachable controls, visible focus, dialog focus return, and meaningful accessible names.
• Keep screenshots or assertions deterministic and free of credentials/private paths.

Non-goals

• Redesigning the product language.
• Expanding protocol capability.
• Modifying Parsar Core.