fix(create-app): default manual setup to npm

[NgoQuocViet2001]

Summary

When create-slidev is run directly with Node, no package manager may be detected. Choosing to install later then produced null install and null run dev in both the generated README and terminal output.

• replace the custom detection logic with package-manager-detector
• fall back to npm only for the manual setup path
• keep the generated README and printed commands aligned

Tests

• pnpm --filter create-slidev build
• pnpm exec eslint packages/create-app/index.mjs packages/create-app/package.json
• pnpm test
• manually ran create-slidev without a package-manager user agent and verified the terminal and generated README use npm

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[pkg-pr-new]

Open in StackBlitz

@slidev/client

npm i https://pkg.pr.new/@slidev/client@2703

create-slidev

npm i https://pkg.pr.new/create-slidev@2703

create-slidev-theme

npm i https://pkg.pr.new/create-slidev-theme@2703

@slidev/parser

npm i https://pkg.pr.new/@slidev/parser@2703

@slidev/cli

npm i https://pkg.pr.new/@slidev/cli@2703

@slidev/types

npm i https://pkg.pr.new/@slidev/types@2703

commit: f45bb98

[antfu]

Let's use https://github.com/antfu-collective/package-manager-detector and fallback to npm. Would you like to update the PR? Thanks

[NgoQuocViet2001]

@antfu Updated — package manager detection now uses package-manager-detector, with npm kept as the fallback for manual setup. Thanks!

[antfu]

Please minimize the code changes. I don't think this file is necessary