core: add -apply-theme CLI argument and runtime theme sync

[khton-tech]

Summary

Adds the -apply-theme <path> command-line option to dynamically apply local .tdesktop-theme files to a running 64Gram instance via IPC at runtime without requiring an application restart or confirmation dialog. Also cleans up the cloud themes list from empty entries and adds an optional settings toggle when running on an Omarchy Linux system.

Details

1. CLI / IPC Command (-apply-theme):

   • Added -apply-theme <path> argument in Launcher::processArguments.
   • When invoked, if an instance is already running, it passes CMD:theme:<path>; via the local UNIX socket.
   • The primary instance applies and saves the palette on the fly via Window::Theme::Apply and Window::Theme::KeepApplied without raising or stealing window focus.
   • If started fresh with -apply-theme, the theme is applied on startup.

2. UI & Cloud Themes Cleanup:

   • Prevents inserting dummy/blank tiles into the cloud themes list when an external system theme is active.
   • Adds a clean toggle in Chat Settings for systems with Omarchy detected (~/.config/omarchy / /usr/share/omarchy) to enable/disable theme synchronization.

Compatibility

• Fully platform-isolated (Platform::IsLinux()), no changes or regressions on Windows/macOS or distros without Omarchy.