refactor: restructure mytoolkit with new subcommands and md-to-pdf improvements

- Add new subcommands: convert, preflight, server, templates, webpage
- Migrate config from bin/config.json to ~/.config/mytoolkit
- Fix expand_bookmarks to modify writer objects instead of reader
- Improve md_to_pdf with CJK bookmark support
- Update README and project metadata

bin/config.py

@@ -3,23 +3,34 @@
 import json
 from pathlib import Path
 
+CONFIG_PATH = Path.home() / ".config" / "mytoolkit" / "env.json"
+_LEGACY_PATH = Path(__file__).parent / "config.json"
+
 
 class Config:
     """Simple config manager for environment variables."""
 
     def __init__(self):
-        self._config_path = Path(__file__).parent / "config.json"
+        self._config_path = CONFIG_PATH
+        self._migrate_legacy()
         self._data = self._load()
 
+    def _migrate_legacy(self) -> None:
+        """One-shot move of legacy bin/config.json into the user config dir."""
+        if _LEGACY_PATH.exists() and not self._config_path.exists():
+            self._config_path.parent.mkdir(parents=True, exist_ok=True)
+            _LEGACY_PATH.rename(self._config_path)
+
     def _load(self) -> dict:
         if self._config_path.exists():
-            with open(self._config_path) as f:
+            with open(self._config_path, encoding="utf-8") as f:
                 return json.load(f)
         return {"vars": {}}
 
     def save(self):
         """Save config to file."""
-        with open(self._config_path, "w") as f:
+        self._config_path.parent.mkdir(parents=True, exist_ok=True)
+        with open(self._config_path, "w", encoding="utf-8") as f:
             json.dump(self._data, f, indent=2, ensure_ascii=False)
             f.write("\n")