feat: invoice preview panel, send modal, and activity timeline

[0xdevcollins]

Closes #70

Summary

SendInvoiceModal

• Pre-filled recipient email (editable) with inline validation
• Custom message textarea (up to 1000 chars) with character counter
• Invoice summary card showing invoice #, customer, total, and due date
• Async onConfirm(invoice, message?) — integrates with existing useSendInvoice hook
• Opens from the "Send Invoice" button in InvoiceDetailSheet; replaces the previous direct-fire behaviour

InvoicePdfPreview

• Full-screen dialog (sm:max-w-4xl, h-[90vh]) wrapping the Cloudinary PDF URL in an <iframe>
• Header action bar: Open, Print, and Download buttons
• useInvoicePdf query hook auto-fetches the URL on open; 5-min stale time avoids redundant regeneration
• Loading spinner while PDF is being generated; error state with fallback download button
• Mobile fallback (< sm) shows "Open in browser" + "Download" since PDF iframes don't work on small viewports
• Triggered via Preview button in the InvoiceDetailSheet header

InvoiceDetailSheet (enhanced)

• Preview button in the sheet header opens InvoicePdfPreview
• Activity timeline — derived from invoice status:
  • Created → Sent → Viewed → Paid (or Overdue / Cancelled branch)
  • Completed steps rendered in primary colour; pending steps shown greyed-out with a "Pending" badge
  • Timestamps shown where available (createdAt, paidAt, dueDate)
• Payment details section — appears for PAID / PARTIALLY_PAID status:
  • Amount received, payment date, balance due (if partial)
• Send Invoice button now opens SendInvoiceModal (no more confirm → immediate send)

Acceptance criteria

• PDF preview shows rendered invoice with merchant branding
• Download PDF and Print buttons work
• Send flow delivers email with optional custom message and updates status
• Activity timeline shows all state changes
• Toast notifications on success/error
• Works on mobile (375px) — PDF preview degrades gracefully

Test plan

• Open a DRAFT invoice → click "Preview" → verify PDF renders in dialog
• Click "Print" — browser print dialog should appear
• Click "Send Invoice" → modal opens with customer email pre-filled
• Add a custom message → Send → verify email received with message
• Status badge in the list updates to SENT after send
• Open a PAID invoice → verify payment details section is visible
• Open a CANCELLED invoice → verify timeline shows Cancelled branch
• Test on 375px viewport — preview shows mobile fallback correctly

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