feat: Add embedded backend packaging for all-in-one deployment

- Add backend/run_server.py entry point for embedded deployment
- Add backend/build.py PyInstaller script for backend packaging
- Modify config.py to support frozen executable paths
- Extend client/config.json with backend configuration section
- Add backend sidecar management in Electron main process
- Add Whisper model download progress reporting
- Update build-client.bat with --embedded-backend flag
- Update DEPLOYMENT.md with all-in-one deployment documentation

This enables packaging frontend and backend into a single executable
for simplified enterprise deployment. Backward compatible with
existing separate deployment mode (backend.embedded: false).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/config.py

@@ -1,9 +1,20 @@
 import os
+import sys
 from dotenv import load_dotenv
 
 load_dotenv()
 
 
+def get_base_dir() -> str:
+    """Get base directory, supporting PyInstaller frozen executables."""
+    if getattr(sys, "frozen", False):
+        # Running as PyInstaller bundle
+        return os.path.dirname(sys.executable)
+    else:
+        # Running as script - go up two levels from app/config.py to backend/
+        return os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+
+
 class Settings:
     # Server Configuration
     BACKEND_HOST: str = os.getenv("BACKEND_HOST", "0.0.0.0")
@@ -49,16 +60,30 @@ class Settings:
         """Return supported audio formats as a set."""
         return set(self.SUPPORTED_AUDIO_FORMATS.split(","))
 
-    def get_template_dir(self, base_dir: str) -> str:
-        """Get template directory path, resolving relative paths."""
+    def get_template_dir(self, base_dir: str | None = None) -> str:
+        """Get template directory path, resolving relative paths.
+
+        Args:
+            base_dir: Base directory for relative paths. If None, uses get_base_dir()
+                     which supports frozen executables.
+        """
+        if base_dir is None:
+            base_dir = get_base_dir()
         if self.TEMPLATE_DIR:
             if os.path.isabs(self.TEMPLATE_DIR):
                 return self.TEMPLATE_DIR
             return os.path.join(base_dir, self.TEMPLATE_DIR)
         return os.path.join(base_dir, "template")
 
-    def get_record_dir(self, base_dir: str) -> str:
-        """Get record directory path, resolving relative paths."""
+    def get_record_dir(self, base_dir: str | None = None) -> str:
+        """Get record directory path, resolving relative paths.
+
+        Args:
+            base_dir: Base directory for relative paths. If None, uses get_base_dir()
+                     which supports frozen executables.
+        """
+        if base_dir is None:
+            base_dir = get_base_dir()
         if self.RECORD_DIR:
             if os.path.isabs(self.RECORD_DIR):
                 return self.RECORD_DIR