Choose the VM keyboard layout and carry the host's timezone into prepared images

[MickLesk]

The Y/Z swap in noVNC happens when the VM's keyboard layout (qm --keyboard, inherited from the datacenter) and the guest's layout differ.

• vm_prompt_keyboard offers Proxmox's layout codes in advanced settings; the default is VM_KEYBOARD, else the datacenter's keyboard: setting, else en-us.
• vm_mark_created sets the layout on the VM (qm set --keyboard), so installer ISOs get the Proxmox side at least.
• vm_prepare_cloud_image writes the matching console keymap into the image (/etc/default/keyboard, /etc/vconsole.conf) and sets the timezone: VM_TIMEZONE, else the host's zone.
• Incus: vm_prompt_keyboard is a no-op, the console layout is the client's there.

Tests: a new case in tests/vm-helpers.sh (42 pass); the generated keymap command was run against a Debian 13 container and produces XKBLAYOUT="de" / KEYMAP=de-latin1, and ch + variant fr / fr_CH-latin1 for fr-ch.

DevScripts: every VM script calls vm_prompt_keyboard in advanced settings; that commit follows once this is on main.

[github-actions]

Try this branch

The engine and the scripts resolve independently, so a production script can
be run against the engine from this PR by setting one variable:

COMMUNITY_SCRIPTS_CORE_URL=https://raw.githubusercontent.com/community-scripts/core/feat/vm-keyboard-timezone \
bash -c "$(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/ct/debian.sh)"

Swap ct/debian.sh for whatever exercises the change.

This PR touches both backends. Worth running on a Proxmox VE and an Incus host.

Run a script from a fork as well

curl -fsSL https://raw.githubusercontent.com/community-scripts/core/feat/vm-keyboard-timezone/tools/run.sh |
  bash -s -- https://raw.githubusercontent.com/YOU/DevScripts/your-branch ct/debian.sh \
             https://raw.githubusercontent.com/community-scripts/core/feat/vm-keyboard-timezone

Note that run.sh is reached through a pipe, so the script it starts inherits
an exhausted stdin. Whiptail is fine — it opens /dev/tty — but a plain read
would see EOF. The single-variable form above does not have that problem.

Useful flags while testing

dev_mode=net logs every engine fetch with status and duration, which is the
quickest way to confirm the branch is really being used. dev_mode=keep stops a
failed build from deleting the container along with the evidence.