[Feature] Implement prompt templates (Q4 2025)

As a user, I want to save and reuse prompt templates, so that I can quickly apply proven prompts without rewriting them each time.

As a user, I want access to a library of prompts, so that I can use best practices and optimize my AI interactions. (Library focus)

As a user, I want to create and manage custom prompts, so that I can standardize common tasks and share effective prompts with my team. (Custom prompts)

As a user, I want quick access to saved prompts, so that I can maintain consistency and efficiency in repetitive AI tasks. (Quick access)