Frontend: announce payment confirmation, limit amount decimals, document locale and form-retention behavior

[jhaydeeee-web]

Closes #914
Closes #913
Closes #912
Closes #911

Summary

c030331: Announce payment confirmation to assistive technology (#914)
Added a persistent visually hidden role="status" live region in InvoicePayment.tsx that announces the payment success message when the receipt replaces the visible status message, which previously went unannounced.

7a52f3d: Document and test fixed-locale token amount formatting (#913)
formatAmount in utils/format.ts already uses an explicit locale (default en-US) rather than the browser locale, so only a maintainer note (docs/drips-notes/913-token-amount-locale.md) and a regression test (tests/formatLocale.test.ts) were added.

b05cfd4: Limit invoice amount input to supported fractional digits (#912)
Added limitFractionDigits in the new utils/amountInput.ts and applied it in the amount branch of handleChange in CreateInvoice.tsx, so input is truncated to the selected token's decimals. A unit test was added.

a888235: Document invoice form preservation after API errors (#911)
CreateInvoice already keeps the entered values when createInvoice fails, since only handleReset clears them. Only a maintainer note (docs/drips-notes/911-invoice-form-preserved.md) was added.

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