Fix scan rate-limiting (CORS errors) + tidy seed input

The "CORS" errors during scan were actually mempool.space rate-limiting:
bursting ~40 address lookups gets some throttled, and throttled responses
lack the access-control-allow-origin header, which the browser reports as a
CORS failure.

- api.js: route all requests through a global throttle (max 2 concurrent,
  250ms min gap between starts, ~4 req/s) with exponential backoff retry so
  occasional throttles recover instead of failing. Unit-tested spacing +
  retry recovery.
- Import pane: drop the "(12 or 24 words)" parenthetical (we generate 12),
  and replace the scrunched ellipsis placeholder with an enumerated
  word1..word12.

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

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