Add How it works page, plainer copy, faster import scan

UI / copy:
- Footer: "made by Adam Soltys" (drop the heart) + a "How it works"
  link on its own line, matching the footer's size.
- New "How it works" page: plain-language basics plus an expandable
  "for the technically curious" section. Routed via ui.screen, with
  Back/logo returning to wherever you opened it from.
- Scrub jargon from the main UI for newcomers: drop BIP39, soften
  "raw hex" -> "code", "txid" -> "transaction ID", "air-gapped" ->
  "offline"; replace em dashes in the intro/keys notes. The technical
  terms now live only in the How it works technical section.

Import / scan performance (fewer requests -> fewer 429s, faster):
- Skip the /utxo fetch for fully-settled, zero-balance addresses
  (they can't hold UTXOs) -- the bulk of a history-heavy wallet.
- Show balance as soon as the address scan finishes, then load UTXOs
  and history in the background.
- Probe the receive and change chains concurrently so their per-hit
  delays overlap.
- History fills in progressively as each address resolves, with a
  loading indicator on the History tab.

i18n.js is now tracked; new English keys fall back to English for the
other languages until translated.

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

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