fix terminal style (#6056)

## Summary

After #5292, at certain browser zoom levels, the xterm terminal did not
fill horizontal space properly, showing a gap with mismatched background
colors (`#171717` viewport vs `black` terminal content).

Fixes https://github.com/Comfy-Org/ComfyUI_frontend/issues/6049

## Problem

The hardcoded `#171717` terminal theme background only affected the
xterm viewport, while terminal content remained black, causing a visible
color mismatch at low zoom levels where the gap was more apparent.

Fixed by keeping original theme when not on desktop distribution.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6056-fix-terminal-style-28c6d73d36508132b521e5767f41d540)
by [Unito](https://www.unito.io)

vitest.setup.ts

@@ -8,6 +8,7 @@ globalThis.__SENTRY_DSN__ = ''
 globalThis.__ALGOLIA_APP_ID__ = ''
 globalThis.__ALGOLIA_API_KEY__ = ''
 globalThis.__USE_PROD_CONFIG__ = false
+globalThis.__DISTRIBUTION__ = 'localhost'
 
 // Mock Worker for extendable-media-recorder
 globalThis.Worker = vi.fn().mockImplementation(() => ({