Add Zero as a cross-platform, privacy-focused email client

Added a new entry for Zero, an open-source, AI-driven email app, to the Cross-Platform section. Zero supports self-hosting, integration with Gmail and other providers, includes a unified inbox, and emphasizes user privacy and extensibility.



Signed-off-by: Alex Grigoras <ahikiwow@gmail.com>

docs/email-clients.md

@@ -24,6 +24,32 @@ OpenPGP also does not support [forward secrecy](https://en.wikipedia.org/wiki/Fo
 
 ## Cross-Platform
 
+### Zero
+
+Zero logo{ align=right }
+
+**Zero** is an open-source, AI-driven, cross-platform email solution that allows you to self-host your email app while supporting integration with Gmail and other major providers. Featuring a privacy-first approach, unified inbox, and customizable UI, it’s built using Next.js and Node.js for modern extensibility.
+
+[:octicons-home-16: Homepage](https://0.email/){ .md-button .md-button--primary }
+[:octicons-eye-16: Privacy Policy](https://github.com/Mail-0/Zero#privacy){ .card-link title="Privacy Policy" }
+[:octicons-info-16: Documentation](https://github.com/Mail-0/Zero#readme){ .card-link title="Documentation" }
+[:octicons-code-16: Source Code](https://github.com/Mail-0/Zero){ .card-link title="Source Code" }
+
+<details class="downloads" markdown>
+<summary>Downloads</summary>
+
+- [GitHub Releases](https://github.com/Mail-0/Zero/releases)
+- [Docker Image](https://github.com/Mail-0/Zero#docker-support)  
+- Self-host via [source code](https://github.com/Mail-0/Zero)
+
+</details>
+
+<div class="admonition warning" markdown>
+<p class="admonition-title">Warning</p>
+Zero depends on self-hosting for full privacy; integrating external providers (like Gmail) subjects some data to those providers' privacy policies.
+</div>
+
+
 ### Thunderbird
 
 <div class="admonition recommendation" markdown>