v1.6.1: Dark mode support and Prompts Manager

1. Custom Prompts Manager

- Sample prompts /tldr, /5q, /wow) for onboarding

2. In-chat prompt dropdown

- Type / in NotebookLM chat to open a filtered prompt picker

- Keyboard navigation (↑↓, Enter/Tab to insert, Esc to close)

- Smart filtering: exact match → prefix → content search

- Categorized and pinned sections in dropdown