feat: add syntax highlighting to .env editor tab (#1459)

Apply Monaco built-in ini language mode to the .env editor tab,
matching the syntax highlighting already used by the FileViewer,
diff previews, and Git source diffs for env content.

- EditorView.tsx: change env tab language from plaintext to ini
- Add EditorView.test.tsx: 3 cases asserting ini/yaml/files tabs
- Docs: update editor.mdx to reflect env tab has highlighting

frontend/src/components/EditorLayout/EditorView.tsx

@@ -524,7 +524,7 @@ export function EditorView(props: EditorViewProps) {
                                             <Suspense fallback={<div className="w-full h-full" aria-busy="true" />}>
                                                 <Editor
                                                     height="100%"
-                                                    language={activeTab === 'compose' ? 'yaml' : 'plaintext'}
+                                                    language={activeTab === 'compose' ? 'yaml' : 'ini'}
                                                     theme={isDarkMode ? 'vs-dark' : 'vs'}
                                                     value={activeTab === 'compose' ? safeContent : safeEnvContent}
                                                     onMount={(editor) => { monacoEditorRef.current = editor; }}