fix(gateway): fall back to junction when symlink unavailable on Windows

ensureExtensionDepsResolvable called symlinkSync without a type argument
and without path normalization. On Windows without Developer Mode or
admin rights, plain symlinkSync throws EPERM; the failure was silently
swallowed, leaving extension-owned packages unresolvable from shared
dist/ chunks and breaking gateway startup.

Extract the link logic into electron/gateway/fs-link.ts:
- linkDirSafe prefers junction on Windows (works without elevation),
  falls back to a plain dir symlink only if junction creation fails
  (e.g. cross-volume).
- normalizeFsPath centralizes the \\?\ extended-length + UNC prefixing
  that was previously an inline helper in config-sync.ts.

Also drop the now-redundant inline fsPath helper in config-sync.ts and
replace the two bare symlinkSync calls with linkDirSafe.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

tests/unit/fs-link.test.ts

@@ -0,0 +1,141 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+const { symlinkSyncMock } = vi.hoisted(() => ({ symlinkSyncMock: vi.fn() }));
+
+vi.mock('fs', async () => {
+  const actual = await vi.importActual<typeof import('fs')>('fs');
+  const mocked = {
+    ...actual,
+    symlinkSync: (...args: unknown[]) => symlinkSyncMock(...args),
+  };
+  return { ...mocked, default: mocked };
+});
+
+vi.mock('node:fs', async () => {
+  const actual = await vi.importActual<typeof import('node:fs')>('node:fs');
+  const mocked = {
+    ...actual,
+    symlinkSync: (...args: unknown[]) => symlinkSyncMock(...args),
+  };
+  return { ...mocked, default: mocked };
+});
+
+describe('fs-link', () => {
+  const originalPlatform = Object.getOwnPropertyDescriptor(process, 'platform');
+
+  beforeEach(() => {
+    symlinkSyncMock.mockReset();
+  });
+
+  afterEach(() => {
+    if (originalPlatform) {
+      Object.defineProperty(process, 'platform', originalPlatform);
+    }
+  });
+
+  function setPlatform(p: NodeJS.Platform): void {
+    Object.defineProperty(process, 'platform', { value: p, configurable: true });
+  }
+
+  describe('linkDirSafe', () => {
+    it('creates a plain dir symlink on POSIX and passes paths through unchanged', async () => {
+      setPlatform('darwin');
+      vi.resetModules();
+      const { linkDirSafe } = await import('@electron/gateway/fs-link');
+
+      linkDirSafe('/src/a', '/dest/a');
+
+      expect(symlinkSyncMock).toHaveBeenCalledTimes(1);
+      expect(symlinkSyncMock).toHaveBeenCalledWith('/src/a', '/dest/a', 'dir');
+    });
+
+    it('prefers junction on Windows and normalizes paths with the extended prefix', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { linkDirSafe } = await import('@electron/gateway/fs-link');
+
+      linkDirSafe('C:/foo/bar', 'C:/baz/qux');
+
+      expect(symlinkSyncMock).toHaveBeenCalledTimes(1);
+      expect(symlinkSyncMock).toHaveBeenCalledWith(
+        '\\\\?\\C:\\foo\\bar',
+        '\\\\?\\C:\\baz\\qux',
+        'junction',
+      );
+    });
+
+    it('falls back to symlink when junction creation throws on Windows', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { linkDirSafe } = await import('@electron/gateway/fs-link');
+
+      symlinkSyncMock.mockImplementationOnce(() => {
+        throw new Error('EXDEV: cross-volume junction not supported');
+      });
+
+      linkDirSafe('C:/foo', 'D:/bar');
+
+      expect(symlinkSyncMock).toHaveBeenCalledTimes(2);
+      const [firstCall, secondCall] = symlinkSyncMock.mock.calls;
+      expect(firstCall[2]).toBe('junction');
+      expect(secondCall[2]).toBe('dir');
+    });
+
+    it('rethrows when both junction and symlink fail on Windows', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { linkDirSafe } = await import('@electron/gateway/fs-link');
+
+      symlinkSyncMock.mockImplementation(() => {
+        throw new Error('EPERM');
+      });
+
+      expect(() => linkDirSafe('C:/foo', 'C:/bar')).toThrow('EPERM');
+      expect(symlinkSyncMock).toHaveBeenCalledTimes(2);
+    });
+  });
+
+  describe('normalizeFsPath', () => {
+    it('passes POSIX paths through unchanged', async () => {
+      setPlatform('linux');
+      vi.resetModules();
+      const { normalizeFsPath } = await import('@electron/gateway/fs-link');
+
+      expect(normalizeFsPath('/a/b/c')).toBe('/a/b/c');
+      expect(normalizeFsPath('')).toBe('');
+    });
+
+    it('adds the \\\\?\\ prefix on Windows for absolute drive paths', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { normalizeFsPath } = await import('@electron/gateway/fs-link');
+
+      expect(normalizeFsPath('C:/a/b')).toBe('\\\\?\\C:\\a\\b');
+    });
+
+    it('adds the UNC extended prefix for UNC paths on Windows', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { normalizeFsPath } = await import('@electron/gateway/fs-link');
+
+      expect(normalizeFsPath('//server/share/file')).toBe('\\\\?\\UNC\\server\\share\\file');
+    });
+
+    it('does not double-prefix already-normalized Windows paths', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { normalizeFsPath } = await import('@electron/gateway/fs-link');
+
+      const already = '\\\\?\\C:\\x\\y';
+      expect(normalizeFsPath(already)).toBe(already);
+    });
+
+    it('leaves relative Windows paths un-prefixed', async () => {
+      setPlatform('win32');
+      vi.resetModules();
+      const { normalizeFsPath } = await import('@electron/gateway/fs-link');
+
+      expect(normalizeFsPath('a/b')).toBe('a\\b');
+    });
+  });
+});