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/utils/font-family.ts

@@ -0,0 +1,22 @@
+import getSystem from './get-system'
+
+const OS = getSystem()
+
+const MONACO_FONT_FAMILY_BASE = [
+  'Fira Code',
+  'JetBrains Mono',
+  'Roboto Mono',
+  '"Source Code Pro"',
+  'Consolas',
+  'Menlo',
+  'Monaco',
+  'monospace',
+  '"Courier New"',
+  '"Apple Color Emoji"',
+]
+
+if (OS === 'windows') {
+  MONACO_FONT_FAMILY_BASE.push('twemoji mozilla')
+}
+
+export const MONACO_FONT_FAMILY = MONACO_FONT_FAMILY_BASE.join(', ')