Add monthlet to Proprietary

[admin-t27d]

Adds monthlet to the Proprietary section.

• https://monthlet.ai — the AI writes the whole note (headings, tables, layout) and saves it as a plain HTML file in a local folder you choose.
• 📖 because notes are stored as plain HTML files on disk, not in a database or a proprietary container. No 🍎/🤖/🔁/🔒 — there is no mobile app, no built-in sync, and no E2EE yet.
• macOS only (Apple Silicon, macOS 13+). Free to use with your own AI API key.
• Added in alphabetical order in the Proprietary section. Not added to Quick Comparison, since that table is for the most popular tools.

Disclosure: I'm the developer.