* feat(nix): add flake module and offline-friendly build
* chore(nix): upgrade to Node.js 24 and pnpm 10
Update devenv.nix to match project dependencies:
- Upgrade Node.js from 22 to 24 (aligns with Dockerfile node:24-bookworm)
- Upgrade pnpm from 9 to 10 (aligns with package.json pnpm@10.22.0)
* test: fix buffer-get test timeout
Replace external URL with mock server endpoint to prevent test timeout.
The test was trying to fetch from http://example.com which is not mocked,
causing it to timeout. Now uses http://rsshub.test/headers which is
properly mocked in the test setup.
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