[P1] Defer `TokenListController.initialize()` (backgroundConnect contention)

## Summary

Defer `TokenListController.initialize()` to run after the critical startup path completes, reducing I/O contention during `backgroundConnect`.

**Parent Epic:** [#6669](https://github.com/MetaMask/MetaMask-planning/issues/6669) — Break Global Re-render Cascade

## Problem

Benchmarking revealed a **10% regression in backgroundConnect** (~12.6ms) caused by `TokenListController.initialize()` running synchronously during controller initialization. This call loads cached token lists from IndexedDB, competing with the critical startup handshake.

**Benchmark Results:**
| Metric | Main | Branch | Change |
|--------|------|--------|--------|
| backgroundConnect | 123.1ms | 135.7ms | +10.2% ⚠ |
| uiStartup | 812.7ms | 757.1ms | -6.8% ✓ |

## Current Code

```typescript
// app/scripts/controller-init/token-list-controller-init.ts
controller.initialize().catch((error: Error) => {
console.error(
'TokenListController: Failed to initialize from storage:',
error,
);
});
```

## Solution

Defer the initialization using `requestIdleCallback` (with `setTimeout` fallback):

```typescript
const deferredInit = () => {
controller.initialize().catch((error: Error) => {
console.error(
'TokenListController: Failed to initialize from storage:',
error,
);
});
};

// Use requestIdleCallback if available, otherwise setTimeout
if (typeof globalThis.requestIdleCallback === 'function') {
globalThis.requestIdleCallback(deferredInit, { timeout: 2000 });
} else {
setTimeout(deferredInit, 0);
}
```

## Files

- `app/scripts/controller-init/token-list-controller-init.ts`

## Acceptance Criteria

- [ ] Extension loads successfully
- [ ] Token list displays correctly after startup
- [ ] backgroundConnect regression eliminated or reduced
- [ ] No regression in existing tests
- [ ] Unit test verifies deferred initialization

## Estimated Effort

0.5 hours

## Labels

`area-performance`, `team-extension-platform`, `P1`

Start with app/scripts/controller-init/token-list-controller-init.ts and inspect where TokenListController.initialize() runs during controller initialization. Add coverage for deferred initialization and run the existing test suite; done means startup still loads successfully, token lists display correctly, and backgroundConnect contention is reduced without test regressions.

Written by the indexing model from the issue text.