**Steps to reproduce:**
- Create a fiscal position T1, with detect automatically
- Create another one T2, without the detect automatically
- When looking at the fiscal positions list, make sure T1 is on top, followed by T2
- Go to the PoS settings, check flexible taxes
- Put T2 as default and in allowed, don't put T1 in allowed
- Go to the PoS, chose a customer
- The fiscal position is T1 even though it's not allowed

**Why the fix:**

Currently, the fiscal position is chosen like this in order:
- A FP specified on the customer's profile
- A FP detected with the detect automatically setting
- The default FP from the PoS settings

When we have a tie, it's the first one in the fiscal positions list that is chosen.

Before this commit, we did not check that the fiscal position was allowed to be used in the PoS, so we just fetched whatever fiscal position fit the best for a given customer and didn't check if we could actually use it.

We now make sure that the fiscal position we try to use is allowed in the current PoS, and if it's not we fall back to the default one.

opw-6032031

Forward-Port-Of: odoo/odoo#271343

### Steps to reproduce:
- Download 'Sales' application
- From 'Configuration' > 'Settings', enable 'Promotions, Loyalty & Gift Card'
- From 'Products' > 'Gift cards & eWallet', Configure an eWallet program with a top-up product
- Have a customer with an existing eWallet balance
- Create a new sale order for that customer and add the eWallet top-up product
- Pay the order using the customer's eWallet
> The order gets discounted by the eWallet, effectively allowing the user to top up their balance
using the balance itself (infinite money glitch).

### Cause of Issue:
When computing the discountable amount for payment programs (like eWallets and gift cards), `_discountable_order` includes the total order amount. However, it did not exclude the program's own top-up products (`trigger_product_ids`) from the discountable lines.

### Fix:
If an order consists solely of top-up products, attempting to apply the eWallet now correctly raises a `UserError` ("There is nothing to discount").

opw-6341410

Forward-Port-Of: odoo/odoo#273419

Steps to produce:
---
- Install the `sales` module.
- Create a product and `enable track inventory.`
- Log in as user with only view access rights in products and also have the sales access rights.
- Create a sale order containing product and confirm it.

Issue:
---
-  An access error is raised during order confirmation.

Root cause:
---
- In [commit], to handle inventory tracking, the `qty_available` field
was moved to `product.product`. Unlike before, this value is increased
or decreased depending on the operation performed.

- As a consequence, creating or updating a sale order triggers a write
to this `qty_available` field on the related product. This
write happens under the current user's permissions, so users who
only have read access to products (but can create/edit sale orders)
hit an `AccessError`, since they lack write access on
`product.product`.


Solution:
---
- Use `sudo()` when accessing the required product quantity information
  to ensure the operation can be completed without requiring additional product
  access rights.

The same issue also occurs when confirming a purchase order.

[commit]: https://github.com/odoo/odoo/commit/ca96992919b11105da44238c3e522f8eec4a740b

opw-6290608
---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

# How to reproduce
- In Settings, enable Variants & Product Reference Price
- Create a published Product with a Sales Price
- Add 2 variants to the Product
- In the product's variant list, select the first one & set Base Unit Count to 0,
- Set the second variant's Base Unit Count to a value > 0
- Go to the Product's page
- Select the second variant

# The issue
The Reference Price is not displayed for the second variant, even though it should since it has a Base Unit Count > 0. Refreshing the page while being on the second variant will prevent the bug from happening.

# Cause
When loading the product's info, we call `_onChangeCombination`. This method is responsible for, among other things, updating the reference price and hiding it if Base Unit Count  = 0 :
https://github.com/odoo/odoo/blob/68f258e99f42693131a5309b3606c3b95f93d824/addons/website_sale/static/src/js/variant_mixin.js#L277-L289

To do that, it will search for an html element with the `.o_base_unit_price` css class. If it does not find it, the reference price will not be updated. The issue is that this element is behind a condition in the template :
https://github.com/odoo/odoo/blob/68f258e99f42693131a5309b3606c3b95f93d824/addons/website_sale/views/templates.xml#L2083

When the first time the template is loaded, if
`combination_info.get('base_unit_price')` is False, then the Reference Price will never be added to the view and will never be found by `_onChangeCombination`.

Since our first variant has Base Unit Count = 0, then `base_unit_price` will equal 0, so `combination_info.get('base_unit_price')` will be evaluated to false.

opw-6367289

---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#274722

**Steps to reproduce:**

- Create a product A, tracked by lots
- Create a kit product, include a component A
- Change the UoM to 0.5
- Go to the PoS, order this kit product
- Also order the component A, with a quantity of 2
- Pay for it, ask for an invoice
- Go to the created picking
- The Demand column is correctly computed and is 0.5
- The Quantity column is wrong and is 2

**Why the fix:**

When getting the data from https://github.com/odoo/odoo/blob/e0d84c7fbb270d0d1f82572daefa96c2978d3785/addons/point_of_sale/models/stock_picking.py#L283 
we always get the component's line, as the move's product is the component, even if it used to be the kit product's move. This is because when exploding a kit's moves, it gets the kit's component as a product instead of keeping the kit product.

This was introducing a weird behavior because we took the quantity from the component line, and not from the kit line, meaning the kit would always have the same quantity as the component.

We now check if the move is actually a kit product's move, and if it is we adapt the qty to correct one by fetching the correct line's qty, and adapting it with the correct UoM.

Changing the line in itself would not work, as the kit itself is not tracked by lots, so we would not enter https://github.com/odoo/odoo/blob/e0d84c7fbb270d0d1f82572daefa96c2978d3785/addons/point_of_sale/models/stock_picking.py#L284 
and the move line would not be correctly created.

opw-6153000

Forward-Port-Of: odoo/odoo#275115
Forward-Port-Of: odoo/odoo#262551

**Problem:**
When creating a new task, the Customer does not follow the project the user selects: once a first project has filled it, selecting another project keeps the previous project's customer.

**Steps to reproduce:**
1. Create a new task and select a project that has a customer.
2. The Customer is set to that project's customer.
3. Select another project configured with a different customer.
4. Observe the Customer keeps the first project's customer.

**Current behavior:**
The Customer keeps the first selected project's customer.

**Expected behavior:**
The Customer follows the selected project and shows its customer.

**Cause of the issue:**
partner_id is filled by _compute_partner_id, which only assigns a partner while the field is empty. Once a project has filled it, selecting another project no longer refreshes the now non-empty Customer.

**Fix:**
Refresh the Customer from the project on project_id change, but only while the task is new (no _origin). An existing task's customer is left untouched, since it may already carry sale order lines, timesheets, materials or worksheets that must not be reset when the project changes.

opw-6315902