feat(invoices): add shipping address to sales invoices

[nikhilb2]

Summary

Adds shipping address support to sales invoices. Users can now:

• Mark "Shipping address same as billing address" (default checked) on the invoice form
• When unchecked, select a saved address from the ledger or enter a new one inline
• New inline addresses are automatically saved to the ledger for future reuse
• Manage saved addresses per ledger from the Ledger detail page
• Invoice PDF renders a "Ship to" section alongside "Bill to" only when shipping differs from billing

Addresses are stored in a new ledger_addresses table. Invoices snapshot the shipping address at creation time (same pattern as existing billing address snapshot).

Type of change

• feat (new feature)

How to test

1. Apply migrations: DATABASE_URL=... python migrate.py up
2. Open a sales invoice, select a ledger — checkbox "Shipping address same as billing address" should appear, checked by default
3. Uncheck it — a dropdown should appear (empty if no saved addresses) plus label/address fields
4. Enter a new address and create the invoice — the address should be saved to the ledger
5. Create another invoice for the same ledger, uncheck shipping — the saved address should appear in the dropdown
6. Go to the Ledger detail page — "Saved addresses" panel should show the saved address with edit/delete actions
7. Download the invoice PDF — "Ship to" section should appear next to "Bill to" with the entered address
8. Create an invoice with the checkbox checked — PDF should show only "Bill to" (no "Ship to")

Checklist

• My code follows the project style and conventions
• I added/updated tests where appropriate
• I ran relevant checks locally
• I verified this does not break existing behavior

Screenshots (if UI changes)

N/A — shipping section appears in invoice composer when checkbox is unchecked; "Ship to" block renders in PDF alongside "Bill to".

Related issue

Closes #