fix(paths): add macOS branch to install_dir() (helper bin folder)

install_dir() (where auto-downloaded helpers like go2rtc land) had Windows and
Linux branches but no macOS one, so on macOS it fell through to a relative "bin"
path — a go2rtc download would land in the current working directory instead of
the app-data folder. Add the macOS branch mirroring resolve_db_path/
resolve_log_dir: ~/Library/Application Support/kite-gc/bin.

Co-Authored-By: Claude Opus 4.8

Author: b14ckyy

src-tauri/src/flightlog/decoder.rs

@@ -58,6 +58,16 @@ pub fn install_dir() -> PathBuf {
                 .join("bin");
         }
     }
+    #[cfg(target_os = "macos")]
+    {
+        if let Ok(home) = std::env::var("HOME") {
+            return PathBuf::from(home)
+                .join("Library")
+                .join("Application Support")
+                .join("kite-gc")
+                .join("bin");
+        }
+    }
     PathBuf::from("bin")
 }