fix(linux): avoid AppImage Wayland graphics crash

[mattenarle10]

Addresses #107.

The AppImage GTK hook can export GDK_BACKEND=x11 even when launched from a Wayland session. That packaging default can trigger WebKitGTK EGL_BAD_PARAMETER before the window is created.

Changes:

• detect Wayland from XDG_SESSION_TYPE or WAYLAND_DISPLAY
• keep the existing WebKit DMA-BUF fallback
• replace only the AppImage-forced x11 backend with wayland,x11
• preserve other user backend choices
• add Rust regression tests for detection and override decisions

Validation:

• cargo fmt --check
• cargo test --manifest-path src-tauri/Cargo.toml --lib
• cargo check --manifest-path src-tauri/Cargo.toml --release --locked
• bun test
• bun run build
• git diff --check

A Linux AppImage smoke test on Manjaro Wayland is still required before closing the issue or releasing.