fix(lite): serve embed entry for Lite routes (#2672)

[sakitam-fdd]

Scope

Port Tencent#2672 onto current main.

• Missing /embed/* routes in Lite now fall back to embed.html instead of the admin SPA index.html.
• Normal SPA fallback remains unchanged.
• Missing embed.html returns 404 with a warning instead of serving the wrong application.
• Includes GET/HEAD/fallback regression coverage.

Current-main three-way merge is clean. Effective diff remains exactly 2 router files.

[coderabbitai]

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