fix: Add cross-platform handling for tray, updater, and icon generation

- Add platform notes for tray startup, shortcut saving, and Linux-only settings
- Handle stat error in processRecording (missing file)
- Select correct binary name based on OS in updater
- Increase tray icon canvas size from 64 to 128 pixels
- Update frontend dist asset hash

main_instance_unix.go

@@ -1,5 +1,13 @@
 //go:build unix && !windows
 
+// ============================================================
+// UNIX-ONLY FILE — This file compiles on Linux (and macOS) but
+// NOT on Windows. It provides Unix domain socket support for
+// single-instance enforcement and IPC communication with
+// helper processes (e.g. GNOME custom shortcuts).
+// The Windows equivalent stub is main_instance_stub.go
+// ============================================================
+
 package main
 
 import (