[18.0][ADD] mail_composer_template_search: New module for inline template search

[BhaveshHeliconia]

This PR introduces a new module to improve the UX of the mail composer template selector.

Key Features

• Inline Filtering: Users can type directly in the dropdown to filter templates by name.
• Full Keyboard Support: Navigate results with ArrowUp/ArrowDown and select with Enter.
• State-Aware: Handles loading states and "no results found" messaging.
• Non-Invasive: Preserves the standard "Search More..." and "Save as Template" options.

Quotations.mp4