Add Koushi to the ecosystem client directory

I would like to propose adding Koushi to the ecosystem client directory. I am the project's maintainer, contributing as an individual.

Koushi is an experimental desktop Matrix client built with Tauri, React, and matrix-rust-sdk. It supports encrypted chat, browser-based OIDC sign-in, threads, and full-text search across encrypted history, including Japanese and other CJK text.

• Repository and documentation: https://github.com/shinaoka/koushi-matrix
• Latest release: https://github.com/shinaoka/koushi-matrix/releases/latest (v0.8.1, published 2026-09-10)
• License: MIT OR Apache-2.0
• Proposed maturity: Alpha
• macOS Apple Silicon is officially supported. Windows and Linux builds are available but untested and unsupported by the maintainer.

I have prepared a client entry and the project's SVG logo following CONTENT.md and the Tesseract entry. The site builds successfully with Zola 0.20.0. A draft PR will make the proposed listing available for review while this proposal is considered.

There is no review deadline. Thank you for considering the addition.

Start with CONTENT.md and the existing Tesseract entry to verify the proposed Koushi listing and SVG logo conventions. Build the site with Zola 0.20.0, then confirm the client entry and logo are available for review in the draft PR.