feat: implement file editor with syntax highlighting and editing

- Install react-simple-code-editor and prismjs dependencies
- Create language detection utility with 50+ file extensions
- Create SyntaxHighlighter component with Prism.js highlighting
- Create CodeEditor component with syntax-highlighted editing
- Create CommitDialog with diff preview and commit message
- Create FileEditor component integrating view/edit/commit flow
- Replace FileViewer with FileEditor in RepoWorkspace
- Add comprehensive CSS styles for editor, highlighter, and dialog
- Support keyboard shortcuts: Ctrl+E (toggle edit), Ctrl+S (save)
- Quality gates: typecheck ✓ lint ✓ build ✓

apps/web/src/components/code-editor.tsx

@@ -0,0 +1,56 @@
+import React, { useEffect } from "react";
+import Editor from "react-simple-code-editor";
+import { highlightCode, loadLanguage } from "../utils/language";
+
+interface CodeEditorProps {
+  value: string;
+  onChange: (value: string) => void;
+  language: string;
+  readOnly?: boolean;
+}
+
+export const CodeEditor: React.FC<CodeEditorProps> = ({
+  value,
+  onChange,
+  language,
+  readOnly = false,
+}) => {
+  useEffect(() => {
+    const highlight = async () => {
+      await loadLanguage(language);
+    };
+    void highlight();
+  }, [language]);
+
+  const hightlightWithLineNumbers = (input: string) =>
+    input
+      .split("\n")
+      .map(
+        (line, i) =>
+          `<div class="editor-line"><span class="editor-line-number">${
+            i + 1
+          }</span><span class="editor-line-content">${highlightCode(
+            line || " ",
+            language
+          )}</span></div>`
+      )
+      .join("");
+
+  return (
+    <div className="code-editor">
+      <Editor
+        value={value}
+        onValueChange={onChange}
+        highlight={hightlightWithLineNumbers}
+        padding={0}
+        className="editor-textarea"
+        textareaClassName="editor-textarea-input"
+        readOnly={readOnly}
+        style={{
+          fontFamily: '"Fira Code", "Monaco", "Courier New", monospace',
+          fontSize: 14,
+        }}
+      />
+    </div>
+  );
+};