feat: add environment variable injection for Docker sandbox

- Add environment field to sandbox config for injecting env vars into container
- Support $VAR syntax to resolve values from host environment variables
- Refactor frontend API modules to use centralized getBackendBaseURL()
- Improve Doraemon skill with explicit input/output path arguments
- Add .env.example file

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

backend/src/config/sandbox_config.py

@@ -22,6 +22,7 @@ class SandboxConfig(BaseModel):
         auto_start: Whether to automatically start Docker container (default: true)
         container_prefix: Prefix for container names (default: deer-flow-sandbox)
         mounts: List of volume mounts to share directories with the container
+        environment: Environment variables to inject into the container (values starting with $ are resolved from host env)
     """
 
     use: str = Field(
@@ -52,5 +53,10 @@ class SandboxConfig(BaseModel):
         default_factory=list,
         description="List of volume mounts to share directories between host and container",
     )
+    environment: dict[str, str] = Field(
+        default_factory=dict,
+        description="Environment variables to inject into the sandbox container. "
+        "Values starting with $ will be resolved from host environment variables.",
+    )
 
     model_config = ConfigDict(extra="allow")