fix: address code review feedback from PR #423

Addresses feedback from gemini-code-assist and coderabbitai reviewers:

## Duplicate Code (High Priority)
- Extract `getEffectiveDefaultEditor` logic into shared `useEffectiveDefaultEditor` hook
- Both account-section.tsx and worktree-actions-dropdown.tsx now use the shared hook

## Performance (Medium Priority)
- Refactor `detectAllEditors` to use `Promise.all` for parallel editor detection
- Replace sequential `await tryAddEditor()` calls with parallel `findEditor()` checks

## Code Quality (Medium Priority)
- Remove verbose IIFE pattern for editor icon rendering
- Pre-compute icon components before JSX return statement

## Bug Fixes
- Use `os.homedir()` instead of `~` fallback which doesn't expand in shell
- Normalize Select value to 'auto' when saved editor command not found in editors
- Add defensive check for empty editors array in useEffectiveDefaultEditor
- Improve mock openInEditor to correctly map all editor commands to display names

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/components/views/board-view/worktree-panel/hooks/use-available-editors.ts

@@ -1,6 +1,7 @@
-import { useState, useEffect, useCallback } from 'react';
+import { useState, useEffect, useCallback, useMemo } from 'react';
 import { createLogger } from '@automaker/utils/logger';
 import { getElectronAPI } from '@/lib/electron';
+import { useAppStore } from '@/store/app-store';
 
 const logger = createLogger('AvailableEditors');
 
@@ -44,3 +45,30 @@ export function useAvailableEditors() {
     defaultEditor: editors[0] ?? null,
   };
 }
+
+/**
+ * Hook to get the effective default editor based on user settings
+ * Falls back to: Cursor > VS Code > first available editor
+ */
+export function useEffectiveDefaultEditor(editors: EditorInfo[]): EditorInfo | null {
+  const defaultEditorCommand = useAppStore((s) => s.defaultEditorCommand);
+
+  return useMemo(() => {
+    if (editors.length === 0) return null;
+
+    // If user has a saved preference and it exists in available editors, use it
+    if (defaultEditorCommand) {
+      const found = editors.find((e) => e.command === defaultEditorCommand);
+      if (found) return found;
+    }
+
+    // Auto-detect: prefer Cursor, then VS Code, then first available
+    const cursor = editors.find((e) => e.command === 'cursor');
+    if (cursor) return cursor;
+
+    const vscode = editors.find((e) => e.command === 'code');
+    if (vscode) return vscode;
+
+    return editors[0];
+  }, [editors, defaultEditorCommand]);
+}