feat: add Invoice PDF export with print and HTML download

[adarsh-yadav1]

Summary

Closes #94

Adds a self-contained Invoice PDF export feature to GovtInvoice. Users can export the current spreadsheet invoice as a styled, print-ready PDF or download it as a standalone HTML file — both with zero extra dependencies.

What Changed

New files

File	Purpose
Govt-Billing-React/src/hooks/useInvoicePDF.ts	Core hook — exportAsPDF() and downloadHTML()
Govt-Billing-React/src/components/InvoicePDF/InvoicePDFModal.tsx	IonModal UI for choosing export format

How It Works

FAB menu → "Export as PDF"
  └── InvoicePDFModal opens
        ├── "Export as PDF"    → printViaIframe()
        │     └── Hidden iframe injected into page
        │           └── iframe.contentWindow.print() fires
        │                 └── User selects "Save as PDF"
        └── "Download as HTML" → Blob + <a download> → .html file

Key implementation decisions

• Hidden iframe instead of window.open — avoids popup blockers entirely, works on localhost and Capacitor WebView
• No DOM mutation for canvas handling — canvas elements are replaced with [Chart — view in app] via regex on the HTML string, avoiding React re-render conflicts (insertBefore NotFoundError)
• Zero new dependencies — uses only browser-native APIs (Blob, URL.createObjectURL, iframe.contentWindow.print())

UI Flow

1. Tap the FAB button (☰) at bottom right
2. Action sheet opens → tap "Export as PDF"
3. InvoicePDFModal opens showing:
   • Current invoice filename
   • Two export options with descriptions
   • Tip about "Save as PDF" in print dialog
4. Choose an option → toast confirms action → modal closes

Screenshots

Testing Checklist

• Export as PDF — print dialog opens via hidden iframe
• Download as HTML — .html file downloads with correct filename
• Modal opens and closes correctly
• Toast confirmation shows after each action
• Works on Chrome (desktop)
• Works on Firefox (desktop)
• Android Capacitor
• iOS Capacitor

No New Dependencies

Uses only browser-native APIs — no jsPDF, html2canvas, or any other package added. package.json is unchanged.