v3.0 - Block height widgets and a single-call API.

* New "Block height" widget mode: shows the latest bitcoin block
  number with the miner name above and a diagonal "stonks-go-up"
  line behind. Block height only ever climbs, so the line is the
  joke (just like BTC mode's flat line says 1 BTC = 1 BTC).
* All widget data now rides on one consolidated /price/summary.json
  endpoint instead of three. Refreshes are 3x lighter on the wire
  while the on-screen experience is identical.
