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