feat: add structured invoice metadata header

[nehayadav827]

Summary

Adds a structured invoice metadata header workflow to the SocialCalc spreadsheet interface.

This PR introduces a frontend-only invoice details form that allows users to generate a structured invoice header directly inside the spreadsheet before entering line items.

What Changed

Added

• Invoice metadata form above the SocialCalc grid for:

  • Invoice No
  • Date
  • Institution Name
  • Bill To
  • GSTIN

Implemented

• Structured invoice header generation directly into the active SocialCalc sheet

• Automatic insertion of starter invoice table headers:

  • Item
  • Description
  • Quantity
  • Rate
  • Amount

UX Improvements

• Added confirmation prompt before replacing an existing invoice header
• Added default invoice date initialization
• Preserved existing spreadsheet save/export workflow using native SocialCalc workbook commands

Why

The current application behaves like a generic spreadsheet with no structured invoice-generation flow.

This PR improves the invoice creation experience by enabling invoice generation from structured user inputs, which directly aligns with the project acceptance criteria:

"Agentic Invoice Co-Pilot successfully generates invoices from user inputs (forms/prompts/files)"

It also provides a stronger foundation for future AI-assisted invoice workflows.

Technical Details

• Uses existing SocialCalc workbook command APIs
• No Firebase/backend/storage changes
• No new dependencies added
• Fully frontend-only implementation

Validation

Verified locally by:

• running git diff --check
• running npm run lint
• testing invoice metadata insertion into the spreadsheet grid
• verifying replacement confirmation behavior

npm run lint passed with one pre-existing warning in app/page.jsx.

Screenshots

Invoice Metadata Form

Shows the newly added structured invoice input form above the spreadsheet grid before invoice generation.

Generated Invoice Header in Spreadsheet

Shows successful insertion of invoice metadata and automatic line-item table headers directly into the SocialCalc sheet after clicking “Add Header”.