[P1] selectors: extract network identity / config / chains 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

~30 network selectors (`getCurrentNetwork`, `getSelectedNetwork`, `getIsMainnet`/`getIsTestnet`, `checkNetworkAndAccountSupports1559`, `getAllEnabledNetworks`, `getChainIdsToPoll`, `getNetworkClientIdsToPoll`, `getIsCustomNetwork`, `getNetworkConnectionBanner`, network-UI appState reads, …). Network base is external. Two entanglements: the `getTokenNetworkFilter` pair (keep in `tokens.ts`, import it) and `getNetworkToAutomaticallySwitchTo` (imports confirmations/active-tab/domains — needs cross-imports).

## Solution

Create `ui/selectors/network.ts`; isolate the entangled `getNetworkToAutomaticallySwitchTo` via explicit cross-imports; keep the token-network-filter pair in `tokens.ts`.

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

- [ ] `network.ts`; entangled selector handled via cross-imports (no cycle); imports repointed directly to the module (no barrel). Tests green.

Start with ui/selectors/selectors.js and map the listed network selectors, keeping the getTokenNetworkFilter pair in tokens.ts. Create ui/selectors/network.ts, inspect the entangled getNetworkToAutomaticallySwitchTo imports, then repoint importing sites directly to domain modules; run the selector and broader test suite to confirm all tests pass without a barrel.

Written by the indexing model from the issue text.