feat(migration): add v4-to-v5 settings migration runner with backup

src/renderer/lib/migrations/v4-to-v5.ts

@@ -0,0 +1,26 @@
+import { z } from 'zod';
+import { settingsSchema } from '@/lib/validators';
+
+export const v4SettingsSchema = z.object({
+  theme: z.enum(['light', 'dark', 'auto']).default('auto'),
+  customCss: z.string().optional().nullable(),
+  recentFiles: z.array(z.string()).default([]),
+  editorFontSize: z.number().min(10).max(28).default(14),
+  keyBindings: z.record(z.string(), z.string()).optional(),
+  snippets: z.array(z.unknown()).default([]),
+});
+
+export function migrateV4ToV5(v4: unknown): z.infer<typeof settingsSchema> {
+  const parsed = v4SettingsSchema.parse(v4);
+  const defaults = settingsSchema.parse({});
+  return {
+    ...defaults,
+    ...parsed,
+    theme: parsed.theme === 'auto' ? 'system' : parsed.theme,
+    customCssPath: parsed.customCss ?? null,
+    recentFiles: parsed.recentFiles,
+    editorFontSize: parsed.editorFontSize,
+    userBindings: parsed.keyBindings ?? defaults.userBindings ?? {},
+    snippets: parsed.snippets,
+  };
+}