refactor: centralize configuration management with type-safe schema

Major changes:
- Add dataclass-based configuration schema with validation (config/schema.py)
- Create centralized config loader and default values (config/defaults.py, config/loader.py)
- Remove duplicate database_config.py, merge into unified structure
- Consolidate browser settings into ERP config
- Add batch_size parameter support to extractor

Bug fixes:
- Fix settings save error by updating config paths (browser.* → erp.*)
- Fix batch_size not being applied in data extraction

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

gui/material_validation_tab.py

@@ -253,7 +253,7 @@ class MaterialValidationTab(ttk.Frame):
             validator = MaterialStatusValidator(
                 username=self.config.get('erp.username'),
                 password=self.config.get('erp.password'),
-                headless=self.config.get('browser.headless', True),
+                headless=self.config.get('erp.headless', True),
                 verbose=True
             )