Fix Linux sandbox launch and SD library path

[chengzeyi]

Motivation

• Prevent crashes on Ubuntu caused by Electron sandbox incompatibility by forcing --no-sandbox for installed launches.
• Ensure the bundled Stable Diffusion shared library is discovered at runtime by setting LD_LIBRARY_PATH.
• Avoid AppImage-related desktop launcher issues by handling launch behavior in the deb installer wrapper.

Description

• Replace post-install symlink with a wrapper script in build/linux/after-install.sh that execs the packaged binary with --no-sandbox, patches the .desktop Exec= line, installs multiple icon sizes, and refreshes the icon cache.
• Remove the AppImage target and the executableArgs entry from the linux section of package.json so the deb installer/wrapper is used for launches.
• Prepend the SD binary directory to LD_LIBRARY_PATH in electron/lib/sdGenerator.ts before calling spawn so bundled shared libraries (e.g. libstable-diffusion.so) are found.

Testing

• No automated tests were run.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Remove existing alternatives symlink before writing wrapper

On upgrades from the previous deb, /usr/bin/${executable} is a symlink created via update-alternatives. Using cat > /usr/bin/${executable} follows that symlink chain and truncates the real target (often /opt/.../${executable}), replacing the binary with the wrapper script contents. This leaves the installed app broken for users who upgrade. You should rm -f /usr/bin/${executable} (and possibly /etc/alternatives/${executable}) before writing the wrapper, or write the wrapper to a temporary file and mv it into place without following symlinks.

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