Add experimental Linux CEF runtime path

[mehmetozguldev]

Summary

This adds an experimental Linux CEF runtime path for Athas while keeping the default Wry runtime path intact for existing builds.

Changes

• Adds a linux-cef Cargo feature that selects Tauri's CEF runtime on Linux.
• Keeps the default tauri-wry feature for current desktop builds.
• Introduces runtime-specific AppHandle aliases across backend crates that need Tauri handles.
• Switches Linux release jobs to the Tauri CEF CLI path and builds with --no-default-features --features linux-cef.
• Removes WebKitGTK from Linux release dependencies and uses GTK/CEF-oriented setup instead.

Validation

• cargo check -p athas
• Verified the Linux CEF feature graph includes tauri-runtime-cef.
• Verified the Linux CEF feature graph no longer includes tauri-runtime-wry or webkit2gtk.

Notes

The full Linux CEF bundle still needs to be validated on an Ubuntu runner because local macOS cross-checking stops at GTK/GLib pkg-config sysroot setup.

Fixes #572