📝 CodeRabbit Chat: Implement requested code changes

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Code changes was requested by @awsl233777.

• feat: 无前端模式 + 前端可指定后端地址 (headless mode & configurable backend) #598 (comment)

The following files were modified:

• web/src/lib/backend-config.test.ts
• web/src/lib/backend-config.ts

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[awsl233777]

This is the right direction for query/hash normalization, but the build-time helper still needs to preserve the runtime URL contract by rejecting non-http(s) protocols.

Current code accepts any parseable URL:

const parsed = new URL(trimmed);
return (parsed.origin + parsed.pathname).replace(/\/+$/, '');

That means VITE_BACKEND_URL=ftp://api.example.com becomes a configured backend fallback, while setBackendUrl('ftp://api.example.com') still correctly throws Backend URL must use http or https. The two entry points should not diverge.

Suggested fix: add the same protocol check to normalizeBackendOrigin() and add a regression test with vi.stubEnv('VITE_BACKEND_URL', 'ftp://api.example.com') expecting same-origin fallback (buildTransportConfig() undefined).