Added custom HTTP headers manager and cookie JSON import

[vgulerianb]

Summary

• Added a Custom Headers tab to create, edit, and clear HTTP header rules (request or response), scoped per-site or globally, applied via declarativeNetRequest
• Added cookie JSON import to the Cookie Editor (pairs with existing export)
• Hardened header rule application: skipped site-scoped rules without a domain and caught DNR update failures so one bad rule cannot break all rules
• Updated README with the new features

Screenshots

[vgulerianb]

@levelsio Can you review this 😅