fix(dashboard): use fs.mkdir for cloudflared local-bin fallback on Windows

Replace execFileAsync("mkdir", ["-p", ...]) with fs.mkdir({ recursive: true })
in the cloudflared install fallback path. The -p flag is Unix-only and breaks
on Windows cmd.exe ("A subdirectory or file -p already exists"). Test mocks
updated to verify the fs.mkdir call instead of the shelled-out mkdir.

The original report covered both this site and packages/engine/src/worktree-hooks.ts;
the latter was already converted to fs.mkdir independently, so only the
dashboard route change is needed.

Co-Authored-By: kenlin8827 <kenlin8827@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/windows-mkdir-compatibility.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix Windows compatibility in cloudflared install fallback by replacing `execFileAsync("mkdir", ["-p", ...])` with `fs.mkdir({ recursive: true })`. The shell-level `-p` flag is Unix-only and breaks installation on Windows cmd.exe with "A subdirectory or file -p already exists". The worktree-hooks fix from the original report was already landed independently.