Detect stuck low-fee txs and stop actively monitoring them

A pending tx is judged 'stuck' once it has waited ~10 blocks (≈100 min) and
its fee rate (fee/vsize) is below the mempool's current floor (minimumFee) —
i.e. it's been purged and won't confirm without a bump. _txSummary now records
vsize and a firstSeen timestamp (persisted in cache) to support this.

Stuck txs are dropped from active monitoring: excluded from the watched-address
set and from the fast-poll trigger, so they don't hold a socket slot or keep us
in the 6s lane. The slow ~25s reconcile still catches one if the floor later
drops and it confirms. History flags them (⚠ stuck) and the detail view explains
it — pointing outgoing ones at the existing fee bump.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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