PDF invoice reports + SMTP email sending

PDF Reports:
- Professional invoice HTML renderer (company header, partner address,
  styled line items, totals, Odoo-purple branding, A4 @page CSS)
- wkhtmltopdf installed in Docker runtime stage for real PDF generation
- Print button on invoice form (opens PDF in new tab)
- Exported HtmlToPDF/RenderInvoiceHTML for cross-package use

SMTP Email:
- SendEmail + SendEmailWithAttachments with MIME multipart support
- Base64 file attachments
- Config via ODOO_SMTP_HOST/PORT/USER/PASSWORD/FROM env vars
- LoadSMTPConfig helper, nil auth for relay servers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

addons/account/models/account_move.go

@@ -949,6 +949,17 @@ func initAccountMove() {
 		return true, nil
 	})
 
+	// action_invoice_print: opens the invoice PDF in a new tab.
+	// Mirrors: odoo/addons/account/models/account_move.py action_invoice_print()
+	m.RegisterMethod("action_invoice_print", func(rs *orm.Recordset, args ...interface{}) (interface{}, error) {
+		moveID := rs.IDs()[0]
+		return map[string]interface{}{
+			"type":   "ir.actions.act_url",
+			"url":    fmt.Sprintf("/report/pdf/account.report_invoice/%d", moveID),
+			"target": "new",
+		}, nil
+	})
+
 	// -- BeforeCreate Hook: Generate sequence number --
 	m.BeforeCreate = func(env *orm.Environment, vals orm.Values) error {
 		name, _ := vals["name"].(string)