feat: implement config service with JSON read/write and safe fallback

- DEFAULT_CONFIG defines output_format, output_quality, output_dir,
  last_garment_dir, last_print_dir
- load_config(): merges file values with defaults; never raises on
  missing, damaged, or non-object JSON root
- save_config(): creates config/ dir as needed; logs error on failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/config_service.py

@@ -1,2 +1,58 @@
+import json
+import logging
+
+logger = logging.getLogger(__name__)
+
+DEFAULT_CONFIG = {
+    "output_format": "PNG",   # PNG or JPG
+    "output_quality": 95,     # JPG quality 1-95
+    "output_dir": "",         # empty = use app output/ dir
+    "last_garment_dir": "",
+    "last_print_dir": "",
+}
+
+_CONFIG_FILENAME = "app_config.json"
+
+
 def load_config():
-    return {}
+    """Load app config from JSON. Returns defaults merged with file values.
+
+    Never raises: missing file → info log + defaults;
+                  damaged file → warning log + defaults.
+    """
+    from services.file_service import get_config_path
+    config_file = get_config_path(_CONFIG_FILENAME)
+
+    if not config_file.exists():
+        logger.info("Config file not found, using defaults: %s", config_file)
+        return dict(DEFAULT_CONFIG)
+
+    try:
+        with open(str(config_file), encoding="utf-8") as f:
+            data = json.load(f)
+        if not isinstance(data, dict):
+            raise ValueError("Config root is not a JSON object")
+        merged = dict(DEFAULT_CONFIG)
+        merged.update(data)
+        logger.info("Config loaded from %s", config_file)
+        return merged
+    except (json.JSONDecodeError, ValueError) as exc:
+        logger.warning("Config file damaged (%s), using defaults: %s", exc, config_file)
+        return dict(DEFAULT_CONFIG)
+    except OSError as exc:
+        logger.warning("Config file unreadable (%s), using defaults: %s", exc, config_file)
+        return dict(DEFAULT_CONFIG)
+
+
+def save_config(data):
+    """Save config dict to JSON. Logs error on failure, does not raise."""
+    from services.file_service import get_config_path
+    config_file = get_config_path(_CONFIG_FILENAME)
+
+    try:
+        config_file.parent.mkdir(exist_ok=True)
+        with open(str(config_file), "w", encoding="utf-8") as f:
+            json.dump(data, f, ensure_ascii=False, indent=2)
+        logger.info("Config saved to %s", config_file)
+    except OSError as exc:
+        logger.error("Failed to save config to %s: %s", config_file, exc)