fix: stop the block counter flickering between networks in live mode - #61

Closes #36.

With "all networks" selected and live mode on, the block counter visibly flipped back and forth — one networks tip, then anothers, then back.

Both live handlers wrote the shared counter unconditionally with whatever event had just arrived:

// onLiveTx
if (blockEl && tx.block_height > 0) blockEl.textContent = fmtNum(tx.block_height);
// onLiveBlock
if (blockEl) blockEl.textContent = fmtNum(block.height);

With independent SSE streams per network and chains at wildly different heights — topaz around 470k, gnoland1 around 3.14M — the display oscillated between them at whatever rate events arrived. onLiveBlock was the bigger offender, since block events are more frequent than transactions.

Fix

Tips are tracked per network (_liveTips) and only ever move forward. What gets displayed is then a rendering decision:

• One network selected — that networks tip. Events from other networks are ignored rather than overwriting it.
• All networks — the highest tip, which is stable and monotonic, with the actual per-network tips in the tooltip. There is no single "current block" across chains, so showing one number and hiding that fact would be worse than showing the breakdown on hover.

Verified in a browser

Interleaving events from two chains, as the feeds actually do:

Monotonic, with gnoland1: 3 162 601 / topaz: 470 102 in the tooltip. With topaz selected, a gnoland1 event leaves the display untouched.

The other live counters (txs, calls, deploys, runs, sends) were already correct: they increment, and their baseline comes from /api/stats for the same network scope, so summing across networks is the right behavior there.