FEAT: Image display support within Zellij (Sixel fallback)

[wowlegend]

Problem Description

Currently, Matcha's image rendering (Kitty/iTerm2 protocols) does not work when running inside Zellij. Zellij currently "swallows" these graphics protocols, causing images to either not appear or fallback to links, even when the underlying terminal (like Ghostty or WezTerm) supports them.

Proposed Solution

1. Sixel Support: Since Zellij supports Sixel graphics, adding a Sixel rendering fallback would allow images to work within Zellij panes.
2. DCS Passthrough: Alternatively, implement "DCS passthrough" sequences (e.g., wrapping the image data in \ePtmux; or similar Zellij-specific escapes) to allow the image data to reach the outer terminal emulator.

Alternatives Considered

Running Matcha outside of a multiplexer works, but breaks the workflow for users who rely on Zellij for session management.

Additional Context

Related Zellij issue: zellij-org/zellij#2814 (Kitty protocol support). Adding Sixel support to Matcha would be the most robust "universal" fix for multiplexed environments.