docs: add settings partial save feature documentation

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

docs/settings-partial-save.md

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+# Settings Partial Save Feature
+
+## Overview
+
+The settings system now implements partial save functionality to prevent unintended overwrites of configuration values.
+
+## How It Works
+
+1. **Field Whitelist**: Only fields exposed in the UI can be modified
+2. **Deep Merge**: Updates are merged with existing config, preserving unmodified fields
+3. **Backup & Rollback**: Config is backed up before save; failures trigger automatic rollback
+
+## Editable Fields
+
+Currently editable via UI:
+- `erp.url` - ERP system URL
+- `erp.username` - ERP login username
+- `erp.password` - ERP login password
+
+## Adding New Editable Fields
+
+To add a new field to the UI:
+
+1. Add field to whitelist in `src/main/services/config/config-manager.ts`:
+
+```typescript
+const UI_EDITABLE_FIELDS: string[] = [
+  'erp.url',
+  'erp.username',
+  'erp.password',
+  'database.dbType',  // Add new field here
+]
+```
+
+2. Add UI input in `src/renderer/src/pages/SettingsPage.tsx`
+3. Update `handleSaveSettings` to include the new field
+
+## API
+
+### savePartialSettings(settings: Partial<SettingsData>)
+
+Saves only the provided fields, preserving all existing configuration.
+
+**Returns:** `{ success: boolean, error?: string }`
+
+**Validation:**
+- Checks whitelist before applying changes
+- Returns error for unauthorized fields
+
+## Error Handling
+
+- **Unauthorized field**: Returns error message listing invalid fields
+- **Save failure**: Automatically restores from backup
+- **Backup failure**: Logs warning, continues with save
+
+## Backup File
+
+Location: `.env.backup` (in project root)
+
+Created before every save operation. Used for rollback on failure.