fix: run frontend npm steps with cwd instead of --prefix in scaffolded build.zig

[mvanhorn]

Summary

In src/tooling/templates.zig, change the generated build.zig's frontend steps (npm install --prefix frontend at ~line 935 and npm --prefix frontend run build at ~line 939) to plain npm install / npm run build with frontend_install.setCwd(b.path("frontend")) / frontend_build.setCwd(b.path("frontend")), and update the templated README snippet (~line 2746) plus the dev-command template (~line 2007) to cd frontend-style or cwd-based invocations consistent with how the runner executes them. Mirror the same change in the four checked-in example apps (examples/next, react, svelte, vue build.zig), which are copies of the template output. Update the inline std.testing assertions in templates.zig (~line 2872) that currently pin the exact "npm", "install", "--prefix", "frontend" argv.

Why this matters

A macOS user hit "No binary found for darwin-x64" from the old zero-native postinstall-download dispatcher, and its suggested remedy (npm run build:native) failed outside the repo. A Windows user reproduced the follow-on failure: the scaffolded app's zig build run executes npm install --prefix frontend, and npm 10.9.3 dies with ENOENT trying to read the app root's nonexistent package.json; upgrading to npm 11 made it work. The original darwin-x64 half is already obsolete: the package is now @native-sdk/cli with per-platform optionalDependencies (@native-sdk/cli-darwin-x64 exists under packages/native-sdk/npm/) and bin/native.js no longer suggests build:native. The remaining reproducible bug in current code is the npm --prefix invocation baked into the scaffold and examples, which breaks on npm < 11 (notably on Windows).

See #56.

Testing

• Happy path: zig build test (or the templates.zig inline tests) passes with the updated argv assertions; generated build.zig contains cwd-scoped npm steps and no --prefix. - Edge case: scaffold an app with native init --frontend next and confirm zig build run installs frontend deps with npm 10.x semantics (no read of app-root package.json). - Edge case: examples/*/build.zig stay in sync with the template (check-framework-sync/CI template checks still pass). - Error path: npm missing from PATH still surfaces the run-step failure for the frontend-install step with a clear failed-command message.

Fixes #56

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