[FIX] stock: ensure safe UoM change for done smls

We fix 2 related UoM issues:

1. Fix quant inconsistency from changing the UoM of Done
   stock.move.lines

Steps to reproduce:
- Enable "Storage Locations" setting
- Create a new "Storable Product" and create a receipt for 1 unit of it
- Validate the 1 unit receieved
- Open "Detailed Operations" of the move and change the stock.move.line
  UoM to dozen.

Expected result: 13 on hand
Actual result: 1 on hand

To fix this:
- prevent users from editing the UoM after the picking is
  done (i.e. unless adding a new stock.move.line and not saving).
- update the write on done logic so stock.move.line UoM changes are
  considering and will update the quant correctly (in case of RPC or
  direct write).

2. Prevent changing UoM of Done stock.move to prevent inconsistent field
values within stock.move and confusion for users

Steps to reproduce:
- Complete a picking (incoming is easiest to see) with a new product
  (i.e. 0 qty) having 1 unit done.
- Unlock picking and add a new stock.move with 1 unit done and save.
- Edit the just added stock.move's UoM from Units to Dozen.
- Check the quantity on hand / Done qty of stock.move after leaving and
  returning to form.

Expected result: 13 On Hand
Actual Result: 2 On Hand and the "Done" qty in the picking is 0.0083
  (i.e. 1/12 of a dozen)

To fix this:
- prevent users from editing the UoM after the picking is done (unless
  adding a new stock.move and not saving)
- if a Done stock.move UoM is uodated, a UserError occurs because there
  is no straightforward way to ensure the quant is updated correctly
  since is handled within the move.line (i.e. has no visibility to its
  move's uom change => changing only UoM and not qty done will result in
  no quant update)

closes odoo/odoo#75621

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>