fix: sync title bar overlay colors with app theme

The Windows title bar overlay (minimize/maximize/close buttons) was
hardcoded to dark colors and did not update when switching themes.

- Add titleBarTheme helper that maps AppearanceTheme to overlay and
  background colors, using nativeTheme for the system preference
- Call applyTitleBarTheme from the setTheme IPC handler so the overlay
  updates immediately on theme change
- Apply persisted theme on startup so the overlay matches from launch

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

src/main/windows/titleBarTheme.ts

@@ -0,0 +1,38 @@
+import { nativeTheme, type BrowserWindow } from 'electron';
+
+import type { AppearanceTheme } from '@shared/domain/tooling';
+
+interface TitleBarColors {
+  backgroundColor: string;
+  overlay: { color: string; symbolColor: string };
+}
+
+const darkColors: TitleBarColors = {
+  backgroundColor: '#09090b',
+  overlay: { color: '#09090b', symbolColor: '#a1a1aa' },
+};
+
+const lightColors: TitleBarColors = {
+  backgroundColor: '#ffffff',
+  overlay: { color: '#ffffff', symbolColor: '#52525b' },
+};
+
+function resolveEffectiveTheme(theme: AppearanceTheme): 'dark' | 'light' {
+  if (theme === 'light') return 'light';
+  if (theme === 'dark') return 'dark';
+  return nativeTheme.shouldUseDarkColors ? 'dark' : 'light';
+}
+
+function colorsForTheme(theme: AppearanceTheme): TitleBarColors {
+  return resolveEffectiveTheme(theme) === 'light' ? lightColors : darkColors;
+}
+
+export function applyTitleBarTheme(window: BrowserWindow, theme: AppearanceTheme): void {
+  const { backgroundColor, overlay } = colorsForTheme(theme);
+  window.setBackgroundColor(backgroundColor);
+
+  // setTitleBarOverlay is only available on Windows (and some Linux WMs).
+  if (typeof window.setTitleBarOverlay === 'function') {
+    window.setTitleBarOverlay(overlay);
+  }
+}