Add manual tax entry on orders and quotes

[ehuelsmann]

On request of @freelock, added manual taxes on orders.

After this change, tax recalculation on the order requires the user to switch the order back to Automatic, which recalculates (and switches back to Manual after saving).

Problems that need addressing:

• When converting an order partially to an invoice (that is, some of the order, but not all), the manual tax table is copied verbatim, resulting in clear incorrect numbers; solutions:
  1. clear all the fields, but keep the manual tax checkmark (which shows an empty table, with no reference of what was in the order)
  2. set the invoice to automatic calculation (which recalculates taxes; which could be wrong, but isn't necessarily; the table equally misses reference to what was on the order)

[Copilot]

Pull Request Overview

Adds support for defining manual tax entries on orders/quotes and ensures these are persisted and reloaded instead of automatically recalculating taxes.

• Wraps existing tax calculation in save() with a manual_tax check and inserts manual tax rows when enabled
• Queries and populates manual tax values in retrieve() if saved entries exist
• Updates Changelog to document the new manual-tax feature and its effect on recalculation

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 3 comments.

File	Description
old/lib/LedgerSMB/OE.pm	Added conditional branches in save()/retrieve() to handle manual taxes
Changelog	Noted ability to manually enter taxes and changed tax recalculation behavior

Comments suppressed due to low confidence (1)

old/lib/LedgerSMB/OE.pm:301

• Consider adding automated tests to cover the new manual-tax entry and retrieval logic to validate both save and load flows.

            if (!$form->{manual_tax}){