Invoice Expiry Date-Time Picker with Timezone Support

[Kingsman-99]

Description

The invoice creation form has a plain date-only input for expiry that ignores time zones, causing invoices to expire at unpredictable moments for creators and recipients in different regions. A date-time picker with explicit timezone selection is required.

Technical Context

src/components/invoice/ExpiryDatePicker.tsx (new) uses react-day-picker for the calendar UI paired with a time input and an IANA timezone selector built from Intl.supportedValuesOf('timeZone'). The selected value is converted to a UTC ISO 8601 string before being stored in src/hooks/useInvoiceForm.ts. src/components/invoice/ExpiryDisplay.tsx shows the expiry in the viewer's local timezone using Intl.DateTimeFormat with the timeZoneName: 'short' option.

Acceptance Criteria

• The expiry field on the creation form opens a calendar with a time input and a timezone dropdown
• The selected expiry is stored as a UTC timestamp in the invoice payload
• The expiry is displayed in the viewer's local timezone on the detail and public status pages
• Selecting a past date-time triggers an inline validation error before form submission
• The timezone dropdown defaults to the browser's detected timezone on first render
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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[Mimah97]

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[grantfox-oss]

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@Mimah97's PR #538 was approved and merged by @Kingsman-99.

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