feat: add Postbode as the lead project

[pataar]

What

Adds Postbode to the projects page as the first entry (order: 0, so the existing entries keep their numbers), with brew install pataar/tap/postbode as the install line, the same way gast is shown.

Why

Postbode is my new project: a Rust mail client whose rules keep the mailbox clean. It's still a work in progress, but it's the project I want to lead with.

Notes for review

• The text describes what v0.1.0 ships today (the IMAP sync engine and CLI) and names what's next: the keyboard-driven GUI, which is still on an unmerged branch, then an MCP server and sending mail. Update the entry when the GUI lands on main.
• The link points to GitHub like the other projects; the docs site at postbode.pataar.nl is an alternative.
• The projects page's meta description doesn't mention a mail client yet.