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>

electron/gateway/fs-link.ts

@@ -0,0 +1,47 @@
+import { symlinkSync } from 'fs';
+import path from 'path';
+
+/**
+ * Normalize a filesystem path for the current platform. On Windows, convert
+ * forward slashes to backslashes and apply the `\\?\` extended-length prefix
+ * for absolute paths so long paths are handled correctly. On POSIX, return
+ * the path unchanged.
+ */
+export function normalizeFsPath(filePath: string): string {
+  if (process.platform !== 'win32') return filePath;
+  if (!filePath) return filePath;
+  if (filePath.startsWith('\\\\?\\')) return filePath;
+  const windowsPath = filePath.replace(/\//g, '\\');
+  if (!path.win32.isAbsolute(windowsPath)) return windowsPath;
+  if (windowsPath.startsWith('\\\\')) {
+    return `\\\\?\\UNC\\${windowsPath.slice(2)}`;
+  }
+  return `\\\\?\\${windowsPath}`;
+}
+
+/**
+ * Create a directory link from `src` to `dest`.
+ *
+ * On POSIX uses a regular symlink. On Windows prefers a junction (which does
+ * not require Developer Mode or administrator privileges) and falls back to
+ * a regular symlink only if the junction attempt fails.
+ *
+ * Throws on POSIX when symlink creation fails. On Windows, both attempts
+ * failing will throw the symlink error — callers guard with try/catch when
+ * link creation is non-fatal (e.g. optional extension dependency linking).
+ */
+export function linkDirSafe(src: string, dest: string): void {
+  const isWin = process.platform === 'win32';
+  const srcP = normalizeFsPath(src);
+  const destP = normalizeFsPath(dest);
+  if (!isWin) {
+    symlinkSync(srcP, destP, 'dir');
+    return;
+  }
+  try {
+    symlinkSync(srcP, destP, 'junction');
+  } catch {
+    // Junction failed (e.g. cross-volume). Try a symlink as a last resort.
+    symlinkSync(srcP, destP, 'dir');
+  }
+}