Fix Linux sandbox crash and add installation docs

[antonlobanovskiy]

Summary

• Fix SUID sandbox crash on Linux — Adds an afterPack hook (build/linux-sandbox-fix.js) that wraps the Electron binary in a shell script passing --no-sandbox, and removes chrome-sandbox. This fixes the fatal setuid_sandbox_host.cc error on Ubuntu 24.04+, Fedora 41, and other modern distros where AppArmor restricts unprivileged user namespaces.
• Remove rpm from default Linux build targets — Building RPM requires rpmbuild which most contributors/users won't have installed, causing the build to fail. AppImage and deb cover the vast majority of Linux users.
• Update README with Linux support — Adds an Installation section documenting FUSE2 dependency for AppImage users (libfuse2t64 on Ubuntu 24.04+), extraction alternative, and updates the features list to reflect Linux picker support.

Context

Tested on Ubuntu 24.04 (kernel 6.17, AppArmor restricting unprivileged userns). The AppImage builds and launches without any --no-sandbox workaround needed from the user.

Related issues: #87, #362

Test plan

• Build on Linux with npx electron-builder --linux AppImage
• Verify the built AppImage launches without sandbox errors
• Verify dist/linux-unpacked/cca is the wrapper script and cca.bin is the real binary
• Verify chrome-sandbox is removed from the unpacked directory
• Verify macOS and Windows builds are unaffected (hook checks electronPlatformName)

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