feat: Web UI for raglogs #7

Summary

Add a web UI to explore incidents: time window selection, explain/timeline/compare views, and links to evidence.

Motivation

• README Roadmap lists a Web UI.
• Lowers friction for demos and for users who prefer browsers over CLI.

Scope (proposal)

• Consume existing HTTP API (prefer after POST /query/timeline and POST /query/compare exist — link those issues).
• Minimal stack aligned with repo conventions (static + FastAPI static mount, or small SPA — decide in implementation).
• Auth story: none for local dev; document reverse-proxy auth for production.

Acceptance criteria

• Local dev instructions: one command (or make target) to run API + UI.
• Core flows: pick window → show explain summary + timeline + compare (as applicable).

Dependencies

• Ideally blocked on or phased after API parity for timeline/compare.