Fix CLN channel confirmations on regtest

[KingParmenides]

Fixes #182.

Summary

• use the local Bitcoind RPC for regtest channel funding transaction status instead of the hard-coded testnet mempool.space endpoint
• choose the mainnet mempool.space endpoint when the stack network is bitcoin, while keeping testnet as the fallback
• persist computed inactive channel confirmations and preserve them across channel polling refreshes

Verification

• npm_config_cache=/private/tmp/npm-cache-swarm npm ci
• npm run build
• npm run check (fails on existing repo-wide type errors such as app/src/api/swarm.ts Cmd union entries and EventTarget.value usages)
• CARGO_HOME=/private/tmp/cargo-home-swarm CARGO_TARGET_DIR=/private/tmp/sphinx-swarm-target PROTOC=/private/tmp/protoc-local/bin/protoc cargo check
• git diff --check -- app/src/api/btc.ts app/src/api/cmd.ts app/src/api/lnd.ts app/src/helpers/bitcoin.ts app/src/lnd/ChannelRows.svelte app/src/lnd/Channels.svelte src/cmd.rs src/conn/bitcoin/bitcoinrpc.rs src/handler.rs