fix(billing): real invoice download with error handling

[OTimileyin]

Closes #191
Title:
fix(billing): real invoice download with error handling

Body:

Summary

• Replaces the stub link.href = '#' with a real fetch â�� Blob â�� object URL download
• Adds per-row loading and error states so failures are surfaced without blocking other rows
• Wraps the invoice table in overflow-x-auto / min-w-[640px] to fix mobile overflow

How the download works

handleDownloadInvoice calls the new downloadInvoice(id) function in client.ts,
which issues an authenticated GET /billing/invoices/:id/download and returns a Blob.
The component then:

1. Creates a temporary object URL via URL.createObjectURL(blob)
2. Attaches it to a transient <a> element and calls .click()
3. Immediately calls URL.revokeObjectURL(url) to release the Blob from memory

The download filename is derived from invoice.invoiceNumber wi
non-[a-zA-Z0-9\-_] characters replaced, so it is never influenced by an
unsanitized server-provided value.

Loading & error handling

• downloadingId: string | null â�� disables and shows a spinner
  fetched; all other rows stay interactive
• downloadErrors: Record<string, string> â�� shows a `role="aler
  below the failing row; cleared on the next successful attempt

Responsive fix

The six-column grid is now wrapped in overflow-x-auto with min-w-[640px] on the
inner container � the table scrolls horizontally on narrow viewp
overflowing.

Tests & coverage

18 tests across 6 describe blocks:

• Rendering â�� empty state, all three status variants (paid / pending / overdue)
• Download success â�� correct invoice id passed, object URL c
• Download error â�� Error message shown, non-Error fallback, per-row isolation,
  no object URL calls on failure
• Loading state â�� button disabled + aria-label updated mid-flight, only the active
  row affected
• Responsive layout â�� overflow-x-auto wrapper and min-w- constraint verified
• Dark theme â�� all theme ternary branches covered

Coverage for InvoicesTab.tsx: **100% statements / branches / f

Security note

• Object URLs are revoked immediately after the click â�� no Blob
• Filenames are sanitized with /[^a-zA-Z0-9\-_]/g â�� no raw server string reaches
  link.download

[Jagadeeshftw]

real invoice download with proper auth and error handling is a solid upgrade over a stubbed button. rebased on latest main and combined it cleanly with the terms api additions in client.ts. thanks!