Implement support for the yearly 1604-C. It has two parts:
- One summary sheet which recaps the monthly 1601-C declarations done
during the year.
- One DAT file containing the detailled income information of all
employees in the company. This is similar to a list of all the forms
2316 for the company.

---

As we are adding support for the 1604-C, we need even
more details from the previous employment of the
employees which joined during the year.

As it starts to make for a lot of fields, we'll move
them to a new model to avoid adding too many columns
on the version/employee model.

---

Currently, the computation of _compute_work_contact_details
is not correct in the Philippines.
The compute should set the VAT, but it is missing from the
depends causing it not to work as expected.

Also add a computed field to store the branch code of the
employee, allowing to easily fetch it when needed in the
declarations.

---

Improve the performance of the declarations test by
batching as much as possible during the setup of the
tests.
Also merge the foreign employee and high earner 2316
tests in once to reduce duplications.

This reduces the time it takes to run the current test
suite by around 25% (calculated on the fly based on logs,
to take with a grain of salt) and simplify the individual
test implementations.

It will also help keep the performances in line when adding
tests for 1604-C which require to create 1601-C and 2316 for
the period.

task-6278100

Preparing a document for signature on a phone was nearly impossible.
The editor relies on a desktop side panel that does not fit on a
small screen: the field types could not be reached or dragged onto
the document, and there was no usable way to manage the signers or
the documents. In practice, users had to wait until they were back
on a computer to prepare and send a signature request.

On small screens, the side panel is now replaced by a bottom sheet
with three tabs (Fields, Signers, Documents). Fields are dragged
from the sheet onto the document — the sheet steps aside during the
drag — and signers and documents are managed like on desktop. The
sheet resizes to a few useful heights and moves up while typing so
the keyboard does not hide what is being edited.

The layout is selected on env.isSmall, which swaps the sidebar for a
new SignTemplateMobileShell while the SignTemplate root and its state
stay shared. The shell is a bottom sheet that snaps between three
heights (peek, half, full) via a handle drag with flick-velocity
detection, and collapses on Escape or the hardware back button; while
an input is focused it expands and pads its content by the keyboard
overlap derived from the visual viewport. The Signers and Documents
tabs extend the desktop sidebar components, inheriting their behavior
and overriding only the template.

task-5149968

Add state income tax withholding for Michigan, Missouri, Kentucky, Utah, South Carolina and Kansas.

Michigan and Kentucky are flat rate states with a personal exemption or standard deduction. Michigan also taxes residents and nonresidents of its 24 cities that have their own income tax. The rate depends on whether the employee lives or works there. Kentucky also has a county occupational tax. 87 of its 120 counties charge one, so we have to support it. We opted to create a new model l10n_us.res.county and add a m2o on res.city. It will be useful for tax reports down the line.

Missouri and South Carolina use progressive brackets. Missouri also withholds a flat 1% earnings tax for employees who live or work in St. Louis or Kansas City.

Utah uses a flat rate reduced by an allowance. That allowance gets smaller as wages go above a threshold, instead of a fixed exemption like other states.

Kansas' brackets and allowances depend on both pay frequency and filing status. Married filing jointly uses different numbers, but single, head of household, and married filing separately all use the same ones.

task-6270166

[IMP] l10n_be_hr_payroll: Holiday attestations rework
Encoding the holiday pay attested by a new hire's previous employer was
split across separate simple/double N and N-1 fields on hr.employee and
an ad-hoc l10n.be.double.pay.recovery.line model, and the amount to
recover and number of days to allocate had to be computed by hand from
the certificate, which is complex and error-prone whenever the
employee's work rate changes.

This commit merges simple and double holiday pay encoding into a single
l10n.be.holiday.attest model on the payroll tab, and compute
automatically, from the certificate (previous year: days assimilated,
working regime, work fraction) and the current contract (current
regime and fraction):
- the number of paid time off days to allocate (LEAVE120 allocation
  and a dedicated unpaid work entry type/leave type),
- the maximum amount to recover, prorated against the employee's work
  rate so that a change of regime between the two employers no longer
  requires a manual computation.

task-5928591