fix: add macOS traffic light inset to sidebar header

Position macOS traffic light buttons (trafficLightPosition) in the
BrowserWindow and add conditional left padding to the sidebar header
so the app logo and title clear the window management controls.

- Create shared platform detection utility (src/renderer/lib/platform.ts)
- Set trafficLightPosition { x: 16, y: 22 } on macOS in createMainWindow
- Apply pl-20 (80px) left padding to the sidebar header on macOS
- Consolidate navigator.platform check from keyboardShortcuts.ts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/windows/createMainWindow.ts

@@ -21,6 +21,9 @@ export function createMainWindow(): BrowserWindowType {
     }),
     backgroundColor: '#09090b',
     titleBarStyle: 'hidden',
+    ...(process.platform === 'darwin' && {
+      trafficLightPosition: { x: 16, y: 22 },
+    }),
     titleBarOverlay: {
       color: '#09090b',
       symbolColor: '#a1a1aa',