[Bug]: plasma-keyboard missing from default KDE Plasma installation — Virtual Keyboard shows "None" in System Settings

## Description

The `plasma-keyboard` package (KDE's virtual keyboard integration for Plasma) is not included in the default CachyOS KDE installation. As a result, **System Settings → Keyboard → Virtual Keyboard** shows only \"None\" with no available options.

`plasma-keyboard` is part of the official `plasma` group in Arch Linux but is absent from the CachyOS default package set.

## Steps to Reproduce

1. Install CachyOS with the KDE Plasma desktop
2. Open System Settings → Keyboard → Virtual Keyboard
3. Observe that only "None" is shown — no virtual keyboard can be selected

## Expected Behavior

At least one virtual keyboard option should be available out of the box (e.g. `plasma-keyboard`, which provides the Qt6 virtual keyboard integration for Plasma).

## Actual Behavior

The virtual keyboard list is empty. Screenshot: [System Settings showing "None"](https://github.com/CachyOS/distribution/issues)

## System Information

- **Distro**: CachyOS Linux
- **Kernel**: 7.0.1-1-cachyos
- **Plasma**: 6.6.4-1.1
- **Qt6-virtualkeyboard**: 6.11.0-1.1 (installed — backend is present but the Plasma integration layer is missing)
- **plasma-keyboard**: not installed

## Package Analysis

```
$ pacman -Si plasma-keyboard
Name : plasma-keyboard
Version : 6.6.4-1
Description : Virtual Keyboard for Qt based desktops
Repository : extra
Depends On : ... qt6-virtualkeyboard wayland ...

$ pacman -Qi plasma-keyboard
error: package 'plasma-keyboard' was not found
```

`qt6-virtualkeyboard` (the Qt backend) is present, but `plasma-keyboard` (the Plasma/KCM integration layer) is missing, which is why KDE System Settings cannot enumerate any virtual keyboard plugin.

## Suggested Fix

Add `plasma-keyboard` to the default CachyOS KDE package list (e.g. as a dependency or optional dependency of `cachyos-kde-settings`, or include it in the KDE ISO package list).

This is particularly important for:
- Touchscreen / tablet users
- Accessibility use cases
- Wayland lock screen (no physical keyboard fallback without it)

## References

- Similar upstream report: https://www.reddit.com/r/kde/comments/qtaii3/why_is_my_virtual_keyboard_list_empty_braknone/
- `plasma-keyboard` Arch package: https://archlinux.org/packages/extra/x86_64/plasma-keyboard/