Offer Thai as an install-time keyboard choice

[kengggg]

Thai is missing from the install-time keyboard picker because the list maps labels to console keymaps and kbd ships no Thai map. Thai typing is still a normal XKB layout (th, Kedmanee by default) — not an IME — and Hyprland already knows what to do with XKBLAYOUT=th: default/hypr/input.lua prefixes us, for non-Latin layouts (Thai is already on that list) and adds grp:alts_toggle.

Selecting Thai (Kedmanee) now persists KEYMAP=us and XKBLAYOUT=th. The console, LUKS prompt, and login password stay Latin. The first session comes up us,th with Left Alt + Right Alt to switch. Caps Lock stays compose.

apply_keyboard in omarchy-provision-owner keeps its existing loadkeys / systemd-firstboot / localectl path. Thai is an extra branch: reuse apply_keyboard us, then omarchy_write_xkblayout. XKB-only picker values are named in OMARCHY_XKB_ONLY_LAYOUTS in setup-form.sh so the ISO configurator can persist them the same way.

Companion: omacom/omarchy-iso#141. Without it, configure_keyboard rejects th and a fresh ISO install stays US. First-boot owner setup on an already-installed machine uses this package alone.

Not in this PR: post-install wizard, Pattachote/Manoonchai, fcitx5, Thai UI locale.

Related: #8050, #7677. Same installer hole as #9299, but Thai is xkb rather than an IME.

Testing

• test/shell.d/setup-form-test.sh — picker maps Thai (Kedmanee) → th; German still → de; English (US) still leads
• test/shell.d/apply-keyboard-test.sh — original persist path still present; omarchy_write_xkblayout rewrites only XKBLAYOUT; that vconsole duals as us,th in default/hypr/input.lua
• test/shell.d/hyprland-keyboard-layout-test.sh — XKBLAYOUT=th → us,th