[P2] util.js: relocate string helpers to string-utils

**Parent Epic:** [#44552 — `util.js` decomposition](https://github.com/MetaMask/metamask-extension/issues/44552)
**Source:** `ui/helpers/utils/util.js`
**Size:** M

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## Problem

`shortenString` (fan-out ~11), `sanitizeString` (bidi/invisible-char escaping — security-relevant), `getAvatarFallbackLetter`. `shortenString` is the base that `shortenAddress` builds on.

## Solution

Move all three to `shared/lib/string-utils.ts`. **Do this before the address-display ticket** so `shortenAddress` can re-import `shortenString` from its new home. Codemod the ~11 `shortenString` import sites.

## Acceptance Criteria

- [ ] Three helpers in `string-utils.ts`; originals deleted; imports rewired. Sequenced before the address ticket. Tests green.

Start in ui/helpers/utils/util.js and inspect the three named helpers, then review shared/lib/string-utils.ts and the roughly 11 shortenString import sites. Rewire the imports and confirm the originals are removed; done means all three helpers are in the new file, address-display work can import shortenString there, and tests are green.

Written by the indexing model from the issue text.