fix: edocument uses draft invoice name instead of final name after submit

[maasanto]

Problem

When a Sales Invoice was submitted, the generated eDocument XML retained the draft invoice name instead of updating to the final invoice name. This occurred because XML generation happened in the before_submit hook, before the invoice received its final name.
Fixes #21

Root Cause

In Frappe's document lifecycle:

1. before_submit hook executes while document still has draft name
2. XML was generated at this point with the draft name
3. After before_submit completes, document gets its final name
4. XML was never updated with the final name

Solution

Split the submit workflow between two hooks:

1. before_submit - Only validates existing edocuments

   • Checks if edocument exists
   • Validates status to potentially block submission
   • Does NOT create or regenerate XML

2. on_submit (new) - Creates/regenerates edocuments with final invoice name

   • Executes after invoice has final name
   • If edocument exists: deletes old XML files and calls generate_xml()
   • If edocument doesn't exist: creates new one with final name

Behavior

Scenario 1: edocument_generation_on_save enabled

• Save → eDocument created with draft name
• Submit → XML regenerated with final name

Scenario 2: edocument_generation_on_save disabled

• Save → No eDocument created
• Submit → eDocument created with final name

Both scenarios now produce edocuments with the correct final invoice name.