feat: add deep merge and validation utility functions to ConfigManager

This commit adds utility functions to support partial settings save functionality:
- isObject: Type guard for plain objects
- deepMerge: Recursively merges objects, preserving unspecified fields
- validateEditableFields: Validates settings against UI editable field whitelist
- UI_EDITABLE_FIELDS: Whitelist of fields modifiable through UI

A failing test is included to verify the deep merge behavior.

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

tests/main/services/config/config-manager.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect, vi } from 'vitest'
+import { ConfigManager } from '@services/config/config-manager'
+import type { SettingsData } from '@types/settings.types'
+
+describe('ConfigManager - deep merge utilities', () => {
+  it('should deep merge objects, updating only specified fields', async () => {
+    const manager = ConfigManager.getInstance()
+    await manager.initialize()
+
+    // Setup initial state
+    const initial: SettingsData = {
+      erp: { url: 'http://old.com', username: 'user1', password: 'pass1', headless: true, ignoreHttpsErrors: true, autoCloseBrowser: true },
+      database: { dbType: 'mysql', server: '', mysqlHost: 'localhost', mysqlPort: 3306, database: 'db', username: 'user', password: '' },
+      paths: { dataDir: '/data', defaultOutput: 'out.xlsx', validationOutput: 'validation.xlsx' },
+      extraction: { batchSize: 100, verbose: true, autoConvert: true, mergeBatches: true, enableDbPersistence: true },
+      validation: { dataSource: 'database_full', batchSize: 2000, matchMode: 'substring', enableCrud: false, defaultManager: '' },
+      ui: { fontFamily: 'Arial', fontSize: 12, productionIdInputWidth: 20 },
+      execution: { dryRun: false }
+    }
+
+    // Load initial settings
+    await manager.saveAllSettings(initial)
+
+    // Partial update
+    const partial = {
+      erp: { url: 'http://new.com' }
+    }
+
+    const result = await manager.savePartialSettings(partial)
+
+    expect(result.success).toBe(true)
+
+    const current = manager.getAllSettings()
+
+    // Updated field
+    expect(current.erp.url).toBe('http://new.com')
+
+    // Preserved fields
+    expect(current.erp.username).toBe('user1')
+    expect(current.database.dbType).toBe('mysql')
+    expect(current.paths.dataDir).toBe('/data')
+  })
+})