[Extension] getTokenPrices re-derives the auth keypair (PBKDF2) on every 30s price poll

[piyalbasu]

Problem

getTokenPrices (v2 path) now routes through the freighter-backend-v2 authed chokepoint (callBackendV2) as of #2880 (#2879). It's the first high-frequency chokepoint caller: the Account view polls token prices every ~30s with useCache: false, so each poll triggers deriveAuthKeypair → mnemonicToSeedSync (bip39 PBKDF2, ~18ms of synchronous, main-thread blocking) to sign the per-request JWT.

Per #2769, the auth keypair is deliberately never cached at rest, so today every poll re-derives it. The other migrated call sites (getDiscoverData, fetchCollectibles, getLedgerKeyAccounts) are one-shot, so this hot-path cost only showed up once getTokenPrices was routed through the chokepoint.

This is correct (no security/functional bug) — just an avoidable recurring main-thread stall.

Options

• A short-lived, in-background keypair memo (e.g. cache the derived keypair in the background service worker for a few seconds, cleared on lock/logout) so a burst of requests reuses one derivation without persisting key material at rest.
• Let the price poll honor the popup cache / back off its interval.
• Move derivation off the main thread.

Any option must preserve the #2769 invariant: no key material persisted at rest, and eviction on lock/logout/expiry.

Context

• Surfaced by the pre-PR whole-system review of feat(auth): route all backend-v2 calls through the authed fetch chokepoint (#2879) #2880.
• Gated behind feat(auth): route all backend-v2 calls through the authed fetch chokepoint (#2879) #2880 (merge that first).