fix(email-composer): hide template buttons when templates are disabled

[Mirrrek]

Summary

Removes the template buttons and keyboard shortcut from EmailComposer when the templatesEnabled feature gate is disabled. See #639 for more information.

Changes

• Use Template & Save as Template buttons are only rendered when templatesEnabled is true.
• Templates keyboard shortcut is disabled when templatesEnabled is false.

Related Issues

Fixes #639

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / Demo

Before (template buttons visible in composer bottom bar):

After (template buttons removed):

Notes for Reviewers

I'm getting lint warnings from other parts of the code and a Next.js runtime error Failed to create address book. None of those are caused by the changes.