fix(deep-link): skip update-desktop-database when not installed (fix #2265)

[btsouth]

Closes #2265

On Linux, register() shells out to update-desktop-database (from the desktop-file-utils package) to refresh the desktop database. That package isn't always installed, e.g. some KDE setups don't need it, so the call fails with ErrorKind::NotFound and the whole registration errors out.

This skips the update-desktop-database step when the command isn't present (logging a warn) and still propagates any other error, matching the existing "binary not found" handling used elsewhere in the workspace (e.g. single-instance). No new dependency is added. xdg-mime is left as-is since it's genuinely required to set the handler.

This is Linux-only. Built and checked on Debian (in a Rust container that does not have desktop-file-utils installed): cargo build, cargo clippy --all-targets -- -D warnings, cargo fmt --check, and cargo test all pass for tauri-plugin-deep-link.

Follow-up (out of scope here): the issue also suggests a KDE fallback (kbuildsycoca6/kbuildsycoca5) to refresh the cache; happy to do that separately if you'd like it.