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

tests/unit/lib/migrations/v4-to-v5.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from 'vitest';
+import { migrateV4ToV5, v4SettingsSchema } from '@/lib/migrations/v4-to-v5';
+
+describe('migrateV4ToV5', () => {
+  test('maps theme auto to system', () => {
+    const out = migrateV4ToV5({ theme: 'auto', editorFontSize: 14 });
+    expect(out.theme).toBe('system');
+    expect(out.editorFontSize).toBe(14);
+  });
+
+  test('drops unknown keys, applies defaults', () => {
+    const out = migrateV4ToV5({ theme: 'light', weirdKey: 1 });
+    expect(out.theme).toBe('light');
+    expect(out.weirdKey).toBeUndefined();
+    expect(out.tabSize).toBe(4); // default
+  });
+
+  test('throws on invalid v4 input', () => {
+    expect(() => migrateV4ToV5({ theme: 42 })).toThrow();
+  });
+
+  test('is idempotent — running twice yields the same v5 output', () => {
+    const v4 = { theme: 'dark', editorFontSize: 16, recentFiles: ['/a.md'] };
+    const a = migrateV4ToV5(v4);
+    const b = migrateV4ToV5(v4);
+    expect(a).toEqual(b);
+  });
+
+  test('exposes the v4 schema for use by main', () => {
+    expect(v4SettingsSchema).toBeDefined();
+  });
+});

tests/unit/main/updater/migration-runner.test.js

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+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+const { MigrationRunner } = require('../../../../src/main/updater/migration-runner');
+
+function tmpDir() {
+  return fs.mkdtempSync(path.join(os.tmpdir(), 'mig-test-'));
+}
+
+describe('MigrationRunner', () => {
+  test('no-op when migration.version is already 5', () => {
+    const dir = tmpDir();
+    fs.writeFileSync(path.join(dir, 'settings.json'), JSON.stringify({ 'migration.version': 5 }));
+    const r = new MigrationRunner({ dir, transform: (v) => v });
+    expect(r.run()).toBe('skipped');
+    fs.rmSync(dir, { recursive: true });
+  });
+
+  test('runs migration on missing v5 marker, writes v5 file, backs up v4', () => {
+    const dir = tmpDir();
+    fs.writeFileSync(path.join(dir, 'settings.json'), JSON.stringify({ theme: 'light' }));
+    const r = new MigrationRunner({ dir, transform: (v) => ({ ...v, theme: 'system', 'migration.version': 5 }) });
+    expect(r.run()).toBe('migrated');
+    expect(JSON.parse(fs.readFileSync(path.join(dir, 'settings.json'), 'utf-8'))['migration.version']).toBe(5);
+    expect(fs.existsSync(path.join(dir, 'settings.v4.bak.json'))).toBe(true);
+    fs.rmSync(dir, { recursive: true });
+  });
+
+  test('returns fresh on missing file', () => {
+    const dir = tmpDir();
+    const r = new MigrationRunner({ dir, transform: () => ({ 'migration.version': 5 }) });
+    expect(r.run()).toBe('fresh');
+    expect(fs.existsSync(path.join(dir, 'settings.json'))).toBe(true);
+    fs.rmSync(dir, { recursive: true });
+  });
+
+  test('returns failed and preserves v4 on transform throw', () => {
+    const dir = tmpDir();
+    fs.writeFileSync(path.join(dir, 'settings.json'), JSON.stringify({ theme: 'light' }));
+    const r = new MigrationRunner({ dir, transform: () => { throw new Error('boom'); } });
+    expect(r.run()).toBe('failed');
+    expect(fs.readFileSync(path.join(dir, 'settings.json'), 'utf-8')).toContain('light');
+    fs.rmSync(dir, { recursive: true });
+  });
+});