Stop input/select fields losing focus

Two causes, both fixed:
- _scanSpMempool emitted on every WS push even when our pending set was unchanged.
  The mainnet mempool churns constantly, so this re-rendered the whole app every
  few seconds, stealing focus mid-type. Now it only emits when our pending SP set
  actually changes.
- render() rebuilds the DOM (replaceChildren), which inherently drops focus. Added
  focus + caret preservation: capture the focused field's index-path and selection
  before the rebuild, restore them after. So any render source (poll, payment
  push, scan) is now invisible to someone typing.

Verified via CDP: typing 'bc1q…' in the Send address, caret at 6, then forcing a
re-render — focus, value, and caret all retained.

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

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