feat(ui): add custom title bar window controls for Windows and Linux

Hide the native title bar on win32/linux (titleBarStyle: 'hidden', frame
untouched so native resize borders and snapping keep working) and render
minimize / maximize-restore / close buttons in the renderer, mirroring the
existing macOS traffic-light setup.

- New WINDOW:* IPC contract (minimize, toggle-maximize, close, get-state)
  handled in window.events.ts, resolved from the sender WebContents;
  close goes through win.close() so window-bounds persistence still runs.
- WINDOW:STATE_CHANGED pushed on maximize/unmaximize/fullscreen so the
  maximize/restore glyph stays correct for OS-triggered changes; controls
  hide while fullscreen.
- WindowControlsComponent mounts once in app-root as a manual popover so
  it stays in the browser top layer above CDK overlays (dialogs,
  multi-EPG) - same behavior as macOS traffic lights.
- Theme-aware via CSS vars (--app-on-surface, --app-hover-overlay);
  Windows-red close hover. Drag regions get right padding through a
  body-level frameless-platform class.
- Gated by RuntimeCapabilitiesService.usesCustomWindowControls; PWA and
  macOS never mount the controls.

Includes unit specs for the component and IPC handlers, an Electron E2E
suite (window-controls.e2e.ts), and a window-chrome section in
docs/architecture/workspace-shell.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/electron-backend-e2e/src/window-controls.e2e.ts

@@ -0,0 +1,109 @@
+import {
+    closeElectronApp,
+    expect,
+    launchElectronApp,
+    test,
+} from './electron-test-fixtures';
+
+// Custom window controls are only rendered on Windows/Linux; macOS keeps
+// the native traffic lights.
+test.describe('Custom window controls', () => {
+    test.skip(
+        process.platform === 'darwin',
+        'macOS uses native traffic lights instead of custom controls'
+    );
+
+    test('@electron renders the window control buttons', async ({
+        dataDir,
+    }) => {
+        const app = await launchElectronApp(dataDir);
+
+        try {
+            await expect(
+                app.mainWindow.getByTestId('window-minimize')
+            ).toBeVisible();
+            await expect(
+                app.mainWindow.getByTestId('window-maximize')
+            ).toBeVisible();
+            await expect(
+                app.mainWindow.getByTestId('window-close')
+            ).toBeVisible();
+            await expect(
+                app.mainWindow.getByTestId('window-maximize-glyph')
+            ).toBeVisible();
+        } finally {
+            await closeElectronApp(app);
+        }
+    });
+
+    test('@electron maximize button toggles the native window state', async ({
+        dataDir,
+    }) => {
+        const app = await launchElectronApp(dataDir);
+
+        try {
+            const isMaximized = () =>
+                app.electronApp.evaluate(({ BrowserWindow }) => {
+                    const mainWindow = BrowserWindow.getAllWindows()[0];
+                    return mainWindow ? mainWindow.isMaximized() : false;
+                });
+
+            await app.mainWindow.getByTestId('window-maximize').click();
+            await expect.poll(isMaximized, { timeout: 10_000 }).toBe(true);
+            await expect(
+                app.mainWindow.getByTestId('window-restore-glyph')
+            ).toBeVisible();
+
+            await app.mainWindow.getByTestId('window-maximize').click();
+            await expect.poll(isMaximized, { timeout: 10_000 }).toBe(false);
+            await expect(
+                app.mainWindow.getByTestId('window-maximize-glyph')
+            ).toBeVisible();
+        } finally {
+            await closeElectronApp(app);
+        }
+    });
+
+    test('@electron reflects window state changes triggered from the main process', async ({
+        dataDir,
+    }) => {
+        const app = await launchElectronApp(dataDir);
+
+        try {
+            await app.electronApp.evaluate(({ BrowserWindow }) => {
+                BrowserWindow.getAllWindows()[0]?.maximize();
+            });
+            await expect(
+                app.mainWindow.getByTestId('window-restore-glyph')
+            ).toBeVisible({ timeout: 10_000 });
+
+            await app.electronApp.evaluate(({ BrowserWindow }) => {
+                BrowserWindow.getAllWindows()[0]?.unmaximize();
+            });
+            await expect(
+                app.mainWindow.getByTestId('window-maximize-glyph')
+            ).toBeVisible({ timeout: 10_000 });
+        } finally {
+            await closeElectronApp(app);
+        }
+    });
+
+    test('@electron minimize button minimizes the window', async ({
+        dataDir,
+    }) => {
+        const app = await launchElectronApp(dataDir);
+
+        try {
+            const isMinimized = () =>
+                app.electronApp.evaluate(({ BrowserWindow }) => {
+                    const mainWindow = BrowserWindow.getAllWindows()[0];
+                    return mainWindow ? mainWindow.isMinimized() : false;
+                });
+
+            await app.mainWindow.getByTestId('window-minimize').click();
+            await expect.poll(isMinimized, { timeout: 10_000 }).toBe(true);
+        } finally {
+            await closeElectronApp(app);
+        }
+    });
+});