feat(linux): improve Wayland support and AppImage integration

[minpeter]

Why

Pot's current Linux AppImage works on X11, but a clean install on a modern GNOME Wayland session exposes several related problems:

• the AppImage bundles the build host's libwayland-client.so.0; on newer hosts this can conflict with the compositor/WebKitGTK stack and abort startup with an EGL allocation error;
• Tauri 1's global shortcut backend does not register shortcuts natively on Wayland, so users must manually configure a desktop shortcut that calls Pot's local HTTP endpoint;
• the current selection crate falls back by mutating process-wide XDG_SESSION_TYPE and GDK_BACKEND, which can affect the rest of the running UI;
• the AppImage has no small user-local integration path for installing its command, icon, launcher, and actions;
• media support depends on host GStreamer plugins that are not consistently present.

This PR makes the AppImage usable as a self-contained Wayland-oriented installation while preserving the existing X11, Windows, and macOS paths.

What changed

AppImage runtime and packaging

• Remove the bundled libwayland-client.so.0 from the completed AppDir and rebuild only through linuxdeploy's AppImage output plugin. Calling the full linuxdeploy pipeline again would rescan the AppDir and copy the incompatible host library back.
• Recreate and sign the Tauri updater archive from the corrected AppImage, so the downloadable and updater artifacts contain the same payload.
• Enable GStreamer bundling for the x86_64 AppImage and install the base/good/libav plugins in the Linux build image.
• Pin the Linux builder to Rust 1.82. The moving rust:bullseye tag now selects a compiler that fails in this Tauri 1/Wry dependency graph; the pinned image makes releases reproducible and was used for the full Rust verification below.

Native Wayland shortcuts

• Use org.freedesktop.portal.GlobalShortcuts through ashpd when running in a Wayland session.
• Convert Pot/Tauri shortcut strings to the XDG shortcut syntax (for example, Ctrl+' becomes CTRL+apostrophe).
• Keep the existing Tauri global shortcut manager unchanged on X11 and other platforms.
• Add command-line actions such as pot --selection-translate. They are forwarded through the existing single-instance plugin, providing a compositor/system-shortcut fallback without depending on curl or a fixed HTTP port.

The portal remains responsible for user consent, so the desktop can display its normal shortcut confirmation dialog.

Selected text handling

• On Linux, try the Wayland primary-selection protocol first and then the X11 primary selection when DISPLAY is available.
• Do not rewrite process-wide session/backend environment variables during fallback.
• Keep the existing selection crate implementation for Windows and macOS.

This intentionally does not claim universal selected-text access on Wayland. If a compositor does not expose the primary-selection protocol, native Wayland clients cannot provide their selection through this mechanism; the documentation recommends copying the text or using clipboard monitoring in that case.

User-local AppImage integration

• Add packaging/linux/install-appimage.sh to install or uninstall the AppImage for the current user.
• Install a stable pot command, desktop entry, icon, and desktop actions for translation/OCR.
• Publish the script beside AppImage artifacts and document the Wayland behavior in the Chinese, English, and Korean READMEs.

No onboarding/default translation-provider configuration is changed by this PR.

Verification

• Repacked the Pot 3.0.7 x86_64 AppImage and verified that libwayland-client.so.0 is absent from the final image.
• Ran the corrected AppImage in an isolated GNOME Wayland session for 10 seconds without the previous EGL/Wayland startup error.
• pnpm build
• cargo +1.82.0 test --manifest-path src-tauri/Cargo.toml in Debian Bullseye with WebKitGTK 4.0: 3 tests passed.
• cargo fmt --check and targeted Prettier check.
• bash -n for the modified packaging scripts.
• Installed and uninstalled the real 3.0.7 AppImage in an isolated temporary HOME; verified the executable, symlink, icon, desktop entry, and all four desktop actions.

Review notes

The two commits are separated by concern: the first contains the AppImage Wayland ABI fix, and the second contains the portal, selection, integration, and build-reproducibility improvements. The PR is opened as a draft so the Linux packaging/portal approach can be discussed before merge.