[P1] selectors: extract legacy token / market-data / detection selectors

**Parent Epic:** [#44566 — `selectors.js` decomposition](https://github.com/MetaMask/metamask-extension/issues/44566)
**Source:** `ui/selectors/selectors.js`
**Size:** L

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## Problem

~26 legacy EVM token selectors, many `@deprecated` in favor of `assets.ts` (`getSelectedAccountTokensAcrossChains`, `getNativeTokenCachedBalanceByChainIdSelector`, `getTokenNetworkFilter`, `getIsTokenNetworkFilterEqualCurrentNetwork`, …) plus live ones (`getTokenList`, `getTokensMarketData`, `getNativeCurrencyImage`, token-scan/security-scan). Overlaps existing `token-scan.ts`.

## Solution

Create `ui/selectors/tokens.ts` (after network + preferences). **Triage `@deprecated` entries — delete-vs-fold-into-`assets.ts` rather than moving blindly.** Route token-scan selectors to the existing `token-scan.ts`. Also absorb the small currency/fiat rate cluster (`getUSDConversionRate*`, `getCurrencyRates`, `getShouldShowFiat`) here.

## Acceptance Criteria

- [ ] `tokens.ts` with live selectors; `@deprecated` entries triaged (deleted or folded into `assets.ts`); imports repointed directly to the module (no barrel). Tests green.

Start in ui/selectors/selectors.js and inventory the legacy, live, currency/fiat, and token-scan selectors described there. Compare assets.ts and the existing token-scan.ts before deciding which deprecated entries are deleted, folded, or routed. Done means ui/selectors/tokens.ts contains the retained selectors, imports point directly to it, and the tests are green.

Written by the indexing model from the issue text.