Add PDF generation for invoices

- Generate both HTML and PDF versions of each customer invoice
- PDFs use printpdf with HTML rendering for consistent styling
- Same layout and formatting as HTML output

Cargo.toml

@@ -7,4 +7,5 @@ edition = "2021"
 askama = "0.15.5"
 chrono = "0.4.44"
 csv = "1.4.0"
+printpdf = "0.9.1"
 serde = "1.0.228"