Add Convert to Estimate feature for invoices

New backend endpoint POST /invoices/{id}/convert-to-estimate that
creates a draft estimate from an invoice, copying items, taxes,
custom fields, and financial data. Frontend wired with dropdown
action, store method, and API service call.

lang/en.json

@@ -467,6 +467,8 @@
     "cloned_successfully": "Invoice cloned successfully",
     "clone_invoice": "Clone Invoice",
     "confirm_clone": "This invoice will be cloned into a new Invoice",
+    "convert_to_estimate": "Convert to Estimate",
+    "confirm_convert_to_estimate": "This invoice will be converted into a new Estimate",
     "item": {
       "title": "Item Title",
       "description": "Description",