Set window icon on Admin Panel (title bar + taskbar)

Embed the blue SyncWarden shield icon in the panel binary and set it
as the window icon via Win32 SendMessage/WM_SETICON on startup.
Uses the syncing (blue) variant as the status-independent panel icon.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/panel/icon_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package main
+
+import "unsafe"
+
+func setWindowIcon(windowPtr unsafe.Pointer) {}