Hardcoded MEMPOOL_API constant in src/lib/bitcoin.ts becomes a baseUrl
parameter on every fetch helper, sourced from a new `esploraBaseUrl`
field on AppConfig (default `https://mempool.space/api`). The wallet,
zap dialogs, on-chain zap verification, and NIP-73 Bitcoin tx/address
pages now read the URL from useAppContext and pass it through, so
self-hosted Esplora deployments (or Blockstream's) work without code
changes. The mempool.space-specific `/v1/prices` extension is still
appended by fetchBtcPrice.
Alex Gleason
474ec6cc99