fix: Prevent validation from running during stats command

- Remove validation logic from calculate_tax_yearly_average()
- Add _auto_download_missing_monthly_data() for silent auto-download
- Fix duplicate validation code in CLI that caused unintended execution
- Separate validation from calculation: --stats only calculates, --validate only validates
- Maintain auto-download functionality for missing data in calculations
- Ensure stats command shows only calculation results without validation output

Root Cause: Validation code was embedded in tax calculation function and duplicated in CLI
Solution: Extract validation from calculation, keep auto-download separate
Result: --stats shows clean output, --validate provides full analysis

Testing: ✅ Stats command clean, ✅ Validation command works, ✅ No type errors

src/cli.py

@@ -556,36 +556,6 @@ def main():
                         print(
                             f"'Jednotný kurz' pro daňové účely podle metodiky ČNB pro {currency_code} za rok {year} nebyl nalezen."
                         )
-        debug_print("HIT: Validation condition")
-        print("VALIDATION: Condition matched!")
-        # Validation command
-        base_threshold = args.change_threshold
-        adaptive = not args.no_adaptive
-
-        if args.currency:
-            # Validate specific currency
-            debug_print(f"Validuji data pro měnu {args.currency}...")
-            results = data_validator.validate_currency_data(
-                args.currency, args.year, base_threshold, adaptive
-            )
-
-            if args.json:
-                output_json(results)
-            else:
-                text_output = data_validator.format_validation_text(results)
-                print(text_output)
-        else:
-            # Validate all currencies
-            debug_print("Validuji data pro všechny měny...")
-            results = data_validator.validate_all_currencies(
-                args.year, base_threshold, adaptive
-            )
-
-            if args.json:
-                output_json(results)
-            else:
-                text_output = data_validator.format_validation_text(results)
-                print(text_output)
     elif args.currency and not args.get_rate:
         # Pokud je zadána měna, ale není zadán --get-rate, vytiskneme poslední dostupný kurz
         # Toto musí být až po --stats, jinak by se --stats nikdy nevykonalo