fix: unify font-family usage across GUI

- Remove hardcoded body font-family from index.scss, use MUI theme dt.font_family instead
- Extract monospace font stack to shared MONACO_FONT_FAMILY constant in utils/font-family.ts
- Add platform check for twemoji mozilla in ip-info-card.tsx
- Consolidate 5 Monaco Editor font-family declarations into single constant

src/components/setting/mods/dns-viewer.tsx

@@ -36,7 +36,7 @@ import { useClash } from '@/hooks/use-clash'
 import { showNotice } from '@/services/notice-service'
 import { useThemeMode } from '@/services/states'
 import type { MonacoEditorInstance } from '@/types/monaco'
-import getSystem from '@/utils/get-system'
+import { MONACO_FONT_FAMILY } from '@/utils/font-family'
 
 const Item = styled(ListItem)(() => ({
   padding: '5px 2px',
@@ -1085,9 +1085,7 @@ export function DnsViewer({ ref }: { ref?: Ref<DialogRef> }) {
             padding: {
               top: 33,
             },
-            fontFamily: `Fira Code, JetBrains Mono, Roboto Mono, "Source Code Pro", Consolas, Menlo, Monaco, monospace, "Courier New", "Apple Color Emoji"${
-              getSystem() === 'windows' ? ', twemoji mozilla' : ''
-            }`,
+            fontFamily: MONACO_FONT_FAMILY,
             fontLigatures: false,
             smoothScrolling: true,
           }}