feat(code): disable editing in diff viewer (#1395)

## Problem

files can be "edited" in the diff viewer (b side or unified view). the changes are not persisted, but users shouldn't be able to manually edit these in the first place

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## [phc-editing-bug.mp4 <span class="graphite__hidden">(uploaded via Graphite)</span> <img class="graphite__hidden" src="https://app.graphite.com/user-attachments/thumbnails/2eb83dcd-d847-4fb6-bbb2-c576e84f2847.mp4" />](https://app.graphite.com/user-attachments/video/2eb83dcd-d847-4fb6-bbb2-c576e84f2847.mp4)

## Changes

updates diff viewer to disable editing, keeps 'revert' functionality working but removes ability to click/type

## notes

i think we probably _should_ allow editing, but since nothing persists right now, it's just confusing so i'm disabling it for now

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## How did you test this?

manually

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Author: adboio

apps/code/src/renderer/features/code-editor/components/CodeMirrorDiffEditor.tsx

@@ -38,7 +38,7 @@ export function CodeMirrorDiffEditor({
   const toggleHideWhitespaceChanges = useDiffViewerStore(
     (s) => s.toggleHideWhitespaceChanges,
   );
-  const extensions = useEditorExtensions(filePath, !onContentChange, true);
+  const extensions = useEditorExtensions(filePath, true, true);
   const options = useMemo(
     () => ({
       original: originalContent,

apps/code/src/renderer/features/code-editor/hooks/useEditorExtensions.ts

@@ -39,7 +39,7 @@ export function useEditorExtensions(
       theme,
       mergeViewTheme,
       EditorView.editable.of(!readOnly),
-      ...(readOnly ? [EditorState.readOnly.of(true)] : []),
+      ...(readOnly && !isDiff ? [EditorState.readOnly.of(true)] : []),
       ...(languageExtension ? [languageExtension] : []),
     ];
   }, [filePath, isDarkMode, readOnly, isDiff, wordWrap]);