feat: add local web UI

[raoabinav]

Evaluator Summary

• Abi added a lightweight local web UI to LEANN's existing FastAPI server so users can inspect indexes and run searches without writing Python or installing a separate frontend toolchain.
• Design choice: the UI is packaged static HTML/CSS/JavaScript served from /, avoiding a frontend build system while still adding index selection, search controls, result rendering, empty states, and error states.
• The implementation includes safe asset path resolution to prevent path traversal and keeps the feature scoped to local leann serve usage rather than a hosted dashboard.
• Quality bar: 3 targeted tests cover static asset serving, path safety, and search API compatibility; this is useful polish, but less differentiated than the indexed search, local assistant, agent, and multimodal retrieval PRs.