[P1] Extract active-tab tracker from `background.js`

**Parent Epic:** [#44543 — `app/scripts/background.js` decomposition](https://github.com/MetaMask/metamask-extension/issues/44543)
**File:** `app/scripts/background.js` → `app/scripts/lib/active-tab/`
**Size:** M

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## Problem

`refreshAppActiveTab` (~1445–1500), `initializeAppActiveTab`, and the `tabs.onActivated` / `tabs.onUpdated` / `windows.onFocusChanged` listeners (~2371–2544) keep `appStateController.appActiveTab` + subject metadata synced to the focused tab. Reads/writes `controller.{appStateController,subjectMetadataController}` only; no counters. The two large tab listeners duplicate `setAppActiveTab`+`addSubjectMetadata` logic that DRYs up on extraction. `refreshAppActiveTab` is also called by the side-panel branch of the connection wiring.

## Solution

`app/scripts/lib/active-tab/active-tab-tracker.ts` exporting `installActiveTabTracker({ getController, isInitialized })` that registers the listeners and returns `refreshAppActiveTab` for the side-panel caller.

## Acceptance Criteria

- [ ] TS module with an injected seam; originals deleted; duplicated listener logic DRY'd.
- [ ] Unit test against mock controller + stubbed `browser.tabs`/`windows`.
- [ ] Active-tab sync parity in a dev build.

Start with app/scripts/background.js, especially refreshAppActiveTab, initializeAppActiveTab, and the tabs.onActivated, tabs.onUpdated, and windows.onFocusChanged listeners. Review the side-panel connection wiring that calls refreshAppActiveTab, then add app/scripts/lib/active-tab/active-tab-tracker.ts with the injected seam and a unit test using mocked controllers and browser tabs/windows. Done means the originals are removed, duplicated listener logic is shared, and active-tab sync matches in a dev build.

Written by the indexing model from the issue text.