composer/settings: use ActionButton iconOnly send; replace deprecated navigator.platform with lib/platform (P1-9, P2-46)

src/renderer/lib/platform.ts

@@ -0,0 +1,11 @@
+export function isMac(): boolean {
+  return window.agentCompany.platform === "darwin";
+}
+
+export function cmdOrCtrlLabel(): string {
+  return isMac() ? "Cmd" : "Ctrl";
+}
+
+export function cmdOrCtrlSymbol(): string {
+  return isMac() ? "⌘" : "Ctrl+";
+}