feat: migrate general settings to core PreferencesController

[fabiobozzo]

Description

This PR moves general settings from the extension-specific PreferencesController to the shared PreferencesController from core to enable cross-platform syncing in future releases.

Changes:

• Added CorePreferencesController integration to MetamaskController
• Created migration 172 to migrate existing general settings (currentLocale, theme, useBlockie, currentCurrency, showNativeTokenAsMainBalance, hideZeroBalanceTokens)
• Updated store actions to route general settings to core controller
• Updated selectors to read from core controller with extension fallback
• Added comprehensive migration tests and TODOs for future cleanup

The change is transparent to users but prepares the foundation for settings sync across extension and mobile.

Changelog

CHANGELOG entry: Prepared general settings for cross-platform sync by moving to shared core controller

Related issues

Depends on: MetaMask/core#6111

Manual testing steps

1. Install extension and go through onboarding
2. Navigate to Settings → General
3. Change language, theme, currency, identicon style, and balance display preferences
4. Verify all settings save and persist correctly
5. Check that settings work the same as before (no functional changes)
6. Verify migration works by checking that existing users retain their settings after upgrade

Screenshots/Recordings

Before

Settings managed by extension PreferencesController only

After

Settings managed by CorePreferencesController with seamless user experience (no visible changes)

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​metamask/​controller-utils@​11.10.0 ⏵ 11.11.0			+1
	@​metamask/​preferences-controller@​17.0.0 ⏵ 18.4.1-preview-32f9004a	+2		+28	+3

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