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)

src/composables/bottomPanelTabs/useTerminal.ts

@@ -5,12 +5,14 @@ import { debounce } from 'es-toolkit/compat'
 import type { Ref } from 'vue'
 import { markRaw, onMounted, onUnmounted } from 'vue'
 
+import { isDesktop } from '@/platform/distribution/types'
+
 export function useTerminal(element: Ref<HTMLElement | undefined>) {
   const fitAddon = new FitAddon()
   const terminal = markRaw(
     new Terminal({
       convertEol: true,
-      theme: { background: '#171717' }
+      theme: isDesktop ? { background: '#171717' } : undefined
     })
   )
   terminal.loadAddon(fitAddon)