feat(window): frameless QuickCaptureWindow centered on primary display

Task 18 of the slice plan. Lazy-creates a 640x280 alwaysOnTop
frameless window centered on the primary display's work area
(1/3 from the top). skipTaskbar to keep it out of the alt-tab
list. Auto-hides on blur so capturing-then-clicking-elsewhere
dismisses cleanly. Singleton pattern; show/hide rather than
recreate to keep the <100ms hotkey latency target.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/windows/quickCaptureWindow.ts

@@ -0,0 +1,40 @@
+import { BrowserWindow, screen } from 'electron';
+import { join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+let win: BrowserWindow | null = null;
+const __dirname = fileURLToPath(new URL('.', import.meta.url));
+
+export function getQuickCaptureWindow(): BrowserWindow | null { return win; }
+
+export function createQuickCaptureWindow(): BrowserWindow {
+  if (win && !win.isDestroyed()) return win;
+  const primary = screen.getPrimaryDisplay();
+  const W = 640, H = 280;
+  const x = Math.round((primary.workArea.width - W) / 2 + primary.workArea.x);
+  const y = Math.round((primary.workArea.height - H) / 3 + primary.workArea.y);
+
+  win = new BrowserWindow({
+    width: W, height: H, x, y,
+    frame: false, show: false, alwaysOnTop: true,
+    skipTaskbar: true, resizable: false,
+    webPreferences: {
+      preload: join(__dirname, '../preload/index.js'),
+      contextIsolation: true, nodeIntegration: false, sandbox: false
+    }
+  });
+
+  if (process.env.ELECTRON_RENDERER_URL) {
+    win.loadURL(`${process.env.ELECTRON_RENDERER_URL}/quickcapture/index.html`);
+  } else {
+    win.loadFile(join(__dirname, '../renderer/quickcapture/index.html'));
+  }
+
+  win.on('blur', () => { if (win?.isVisible()) win.hide(); });
+  return win;
+}
+
+export function showQuickCapture(): void {
+  const w = createQuickCaptureWindow();
+  w.show(); w.focus();
+}