Refactor folder structure

[Prithviraj2003]

Fixes #27

Refactor the codebase into a modular structure.

• Create modules/invoice directory and move all invoice-related files from src/components/ui/invoice to modules/invoice.
  • Move BillingInfo.tsx, EntriesTable.tsx, EntryRow.tsx, GenerateInvoice.tsx, InvoiceFooter.tsx, InvoiceHeader.tsx, TaxDetail.tsx, and TaxDetailsTable.tsx to appropriate subdirectories within modules/invoice.
• Create modules/auth directory for future authentication-related files.
• Update imports in src/app/generate/page.tsx to reflect the new modular structure.
• Delete the original files from src/components/ui/invoice.

[aayushchugh]

Looks amazing, ping me once it's ready to be merged @Prithviraj2003