[P2] selectors: extract snaps registry / metadata / interface selectors

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

---

## Problem

~30 snap selectors (`getSnaps`, `getSnap`, `getSnapsMetadata`, `getSnapMetadata`, `getSnapRegistryData`, `getAllSnapAvailableUpdates`, `getInterface(s)`, `getSnapInsights`, `getKeyringSnapAccounts`, `getSnapInstallOrUpdateRequests`, …). Depends on the subject/metadata base now in `permissions.ts` + accounts + locale.

## Solution

Create a top-level `ui/selectors/snaps.ts` (**last** — needs the permissions subject base). Note: the existing `snaps/` dir holds `accounts.ts`/`address-book.ts`, not snap selectors — verify before colliding. Consider moving generic `getLocale` to `app-ui-state.ts` instead.

Migration seam (no barrel): repoint every site importing the moved selectors to `ui/selectors/` directly (a `ts-morph` codemod that splits any `../selectors` barrel imports into per-module imports) and delete them from `selectors.js` in the same PR — no re-export facade, per the no-barrel-files convention.

## Acceptance Criteria

- [ ] `snaps.ts`; depends on `permissions.ts` subject base; imports repointed directly to the module (no barrel). Tests green.

Start with ui/selectors/selectors.js and the listed snap selectors, then read ui/selectors/permissions.ts for the subject and metadata base. Create ui/selectors/snaps.ts, check whether getLocale belongs in app-ui-state.ts, and repoint imports directly to the domain module without a barrel; done means the selectors are removed from selectors.js and tests are green.

Written by the indexing model from the issue text.