Web explorer UI (read-only block explorer)

[Bortlesboat]

Summary

Add a lightweight web UI at `/explorer` that lets users browse blocks, transactions, and addresses visually — powered entirely by the existing API endpoints.

Why

• Demonstrates the API's capabilities visually
• Useful for node operators who want a quick GUI
• Differentiator: self-hosted block explorer in one pip install

Proposed approach

• Single-page app served as static HTML (no build step)
• Fetches data from the API's own endpoints
• Dark theme matching the landing page
• Search bar: paste a txid, block hash/height, or address

Pages

• Block detail (header, tx list, stats)
• Transaction detail (inputs, outputs, fee, size)
• Address summary (balance, UTXOs)
• Dashboard (latest blocks, mempool stats, fee chart)