[Linuxuser67]

Summary

Prepare the next 6.0.2 beta with the merged FFmpeg installer compatibility fix.

• Bump the Cargo package, Cargo.lock root package, and Meson project versions to 6.0.2-beta.5.
• Add the newest-first AppStream entry as 6.0.2~beta.5, dated 2026-10-20.
• Note that FFmpeg/ffprobe installation now recognizes BtbN's current checksum-bearing Linux stable-branch builds.

Validation

• Verified Cargo.toml, the grab package in Cargo.lock, meson.build, and the first AppStream release entry all declare beta.5.
• No dependencies changed, so build-aux/cargo-sources.json does not need regeneration.
• CI should run the normal formatting, Clippy, Rust/GTK tests, vendor-sync, structure, and audit checks.

Release gate

After this PR is merged and CI passes, create and publish tag v6.0.2-beta.5 as a prerelease on the merged versioned commit. The published-release workflows should verify the tag/version match and build the Flatpak bundle and x86_64/aarch64 tarballs.