fix(browse): use --outdir for Windows node server build

[garrett-fox]

Problem

On Windows, bash browse/scripts/build-node-server.sh fails with:

```
error: cannot write multiple output files without an output directory
```

because recent `bun build` (Bun v1.3.11) emits a native asset (`ngrok.win32-x64-msvc-*.node`, ~10 MB) alongside the main server bundle. When bun has multiple outputs, it rejects the `--outfile` flag.

The script crashes before producing `server-node.mjs`, so `browse.exe` can't start on Windows and every `$B` command dies with:

```
error: server-node.mjs not found. Run `bun run build` to generate the Windows server bundle.
```

Every fresh `./setup` or `gstack-upgrade` on Windows leaves `/browse` broken until someone rebuilds by hand.

Fix

Replace `--outfile` with `--outdir`, then rename the emitted entry bundle to `server-node.mjs` before the existing perl substitutions and polyfill-header injection run. All downstream steps are unchanged.

```diff

• --outfile "$DIST_DIR/server-node.mjs" \

• --outdir "$DIST_DIR"
  --external playwright
  ...
  +mv "$DIST_DIR/server.js" "$DIST_DIR/server-node.mjs"
  ```

The ngrok native asset lands in `dist/` as a sibling file; `browse.exe` doesn't reference it, and it's already covered by the existing `browse/dist/` line in `.gitignore`, so it won't get committed.

Verification

Verified end-to-end on Windows 11 / Bun v1.3.11:

1. `rm -f browse/dist/server-node.mjs browse/dist/bun-polyfill.cjs browse/dist/ngrok*.node` — reset state
2. `bash browse/scripts/build-node-server.sh` — prints `Node server bundle ready: .../browse/dist/server-node.mjs`
3. `head -8 browse/dist/server-node.mjs` — polyfill header correctly injected (`createRequire`, `fileURLToPath`, `dirname`, `__browseNodeSrcDir`, `createRequire(... "./bun-polyfill.cjs")`)
4. `bun run build` — full pipeline succeeds (all dist binaries compile, `.version` written)
5. `browse goto https://example.com && browse js "document.title"` → `Example Domain` ✓

No changes needed to `package.json`, `setup`, or any caller — the script is the only thing that had to change.

Not in scope

• The parallel macOS-only `scripts/build-app.sh` line 31 also uses `bun build --compile --outfile`, but that's `--compile` mode (single standalone binary, no side assets) and not triggered on Windows, so it's unaffected.