fix(packaging): stop shipping frontend-contract in Linux bundles

tauri-cli's get_binaries() adds every file in src-tauri/src/bin/ as a
bundle binary, and the deb/rpm packagers copy each one to /usr/bin, so
the dev-only bindings generator ended up as /usr/bin/frontend-contract
(1.0 MB) in release packages and broke downstream repackaging (Copr:
"Installed but unpackaged file(s) found"). There is no bundler option
to exclude a binary, so move the generator to an example target, which
cargo keeps dev-only and get_binaries() never scans, and invoke it via
'cargo run --example'.

Ref https://github.com/clash-verge-rev/clash-verge-rev/pull/8131#issuecomment-6137907930

src-tauri/src/bin/frontend-contract.rs

@@ -1,12 +0,0 @@
-fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let mut args = std::env::args_os().skip(1);
-    let output = args
-        .next()
-        .map(std::path::PathBuf::from)
-        .unwrap_or_else(|| std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../src/services/bindings.ts"));
-    if args.next().is_some() {
-        return Err(std::io::Error::new(std::io::ErrorKind::InvalidInput, "expected at most one output path").into());
-    }
-    app_lib::ipc::builder().export(specta_typescript::Typescript::default(), output)?;
-    Ok(())
-}