feat(plasma): support lock-screen input panels

[inayayousfi]

Written by inayayousfi, typed by gpt-5.6-sol running in OpenCode.
Every call here is inayayousfi's, and no agent acted on its own.

Summary

Add Plasma Login Manager and KDE lock-screen support through KWin's Wayland input-panel protocol.

The secure runtime keeps one process alive across lock cycles while starting and stopping the keyboard backend and window resources with the lock state. Runtime events and commands preserve the existing separation between platform integration, window orchestration, and reusable components.

Setup now manages the Plasma input-method launcher, KWin configuration, systemd override, and the required lock-screen QML block. It validates the Plasma Login Manager structure and supports Plasma versions from 6.7.0 up to, but not including, 7.0.0.

Window opacity now uses a content opacity effect on Wayland and native window opacity elsewhere through one runtime window API.

Testing

• .venv-windows/Scripts/python.exe -m unittest discover -s tests: 194 passed, 1 skipped
• Flake8 F,E9,W6
• git diff --check
• Built the Linux payload from the branch and installed it in a clean Fedora 44 VM through the CLI
• Verified every Plasma Login Manager and uinput status check on Plasma 6.7.4
• Verified login-screen rendering at the configured 0.85 Wayland opacity, real OSK key input, and successful login
• Verified the desktop KWin input-method process and user autostart process after login
• Verified two lock/unlock cycles with the final lifecycle fixes: the secure keyboard had no close control, latched Shift state cleared across unlock, and OSK input worked after relock