feat(appearance): apply UI zoom level to terminal and Monaco editors

Extend the existing appearance zoom setting so it also scales xterm.js
terminals and Monaco editors (diff viewer, three-pane merge editor, file
workspace editor), which previously rendered at a hard-coded 13px
regardless of zoom.

- Terminals read zoom at init and update term.options.fontSize live on
  zoom change, refitting after a double rAF so xterm's renderer has
  recomputed cell metrics. Font size is rounded to an integer to avoid
  subpixel blur in the canvas renderer.
- Monaco editors derive fontSize from zoomLevel; three-pane merge
  editor memoizes its options object to avoid redundant updateOptions
  calls on unrelated re-renders.

src/components/diff/diff-viewer.tsx

@@ -6,8 +6,11 @@ import { ChevronLeft, ChevronRight } from "lucide-react"
 import type { DiffOnMount } from "@monaco-editor/react"
 import type { editor as MonacoEditorNs } from "monaco-editor"
 import { defineMonacoThemes, useMonacoThemeSync } from "@/lib/monaco-themes"
+import { useZoomLevel } from "@/hooks/use-appearance"
 import { cn } from "@/lib/utils"
 
+const EDITOR_BASE_FONT_SIZE = 13
+
 import "@/lib/monaco-local"
 
 const MonacoDiffEditor = dynamic(
@@ -36,6 +39,7 @@ export function DiffViewer({
   className,
 }: DiffViewerProps) {
   const editorTheme = useMonacoThemeSync()
+  const { zoomLevel } = useZoomLevel()
   const diffEditorRef = useRef<MonacoEditorNs.IStandaloneDiffEditor | null>(
     null
   )
@@ -187,7 +191,7 @@ export function DiffViewer({
             renderSideBySide: true,
             renderSideBySideInlineBreakpoint: 0,
             automaticLayout: true,
-            fontSize: 13,
+            fontSize: (EDITOR_BASE_FONT_SIZE * zoomLevel) / 100,
             minimap: { enabled: false },
             scrollBeyondLastLine: false,
             renderOverviewRuler: false,