feat: add Esc keyboard hint support for dialogs

[opascal221-design]

Summary

• add a reusable keyboard hint (Esc to close) inside dialog content for open dialog contexts
• render hint as an absolute, non-interactive overlay so layout flow is unchanged
• add showEscapeHint opt-out prop on DialogContent for contexts where Escape dismissal is intentionally blocked
• wire opt-out in guarded dialog states (TradeDialog while submitting, PendingTxModal while blocked/loading)
• add focused unit tests covering default visible hint and disabled-hint behavior

Validation

• npm test -- src/components/ui/dialog.test.tsx (fails locally: vitest not found in this clone environment)

Closes #223
Closes #224
Closes #227
Closes #229

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