broadcaster: say "last mined block", and show the chain tip beside it

[rsantacroce]

Why

#95 added Last block: #970,851 to the snapshots. That was the newest block the pool mined, but it read as the chain's height. With the network at 970,861, an operator took it to mean the pool's node had fallen ten blocks behind or reorged. The node was in fact at the tip, matching the explorer and its peers.

What

• The line is renamed to Last mined block. It is still the newest pending or confirmed block from /api/blocks.
• A new Chain tip line gives the network's latest block, mined by anyone, as the pool's node sees it. It comes from /api/status's node.tip_height and tip_observed_at, and is left out while the dashboard has no node status yet.

Both lines appear in the live message, the periodic update and /pool_status. Rendered from pool.beta.avonpool.xyz's live API:

🟢 avonpool_beta — status

Hashrate now: 2.05 PH/s (5m)
Hashrate: 2.01 PH/s (1h) · 1.99 PH/s (24h)
Active workers (24h): 1
Shares (24h): 34,767 accepted · 77 rejected (0.22%)
Best share (24h): 66.89M
Last share: 3s ago
Chain tip: #970,861 · 21m ago
Last mined block: #970,851 · 3h 5m ago
Blocks (all time): 60

Tests

npm test in broadcaster/: 34 pass. One test is new: the chain tip line appears with node status and is left out without it. The /pool_status test now checks both lines in order, and the existing assertions use the new label.

🤖 Generated with Claude Code