Configure v1.6.0 display across application

- backend/app/core/config.py: Add APP_VERSION = '1.6.0' setting
- backend/app/main.py: Update health and root endpoints to show APP_VERSION
- frontend-internal/Sidebar.svelte: Add version display in sidebar footer
- frontend-client/+layout.svelte: Add version in page footer
- All endpoints now return both app_version (1.6.0) and api_version (v1)

backend/app/core/config.py

@@ -27,6 +27,7 @@ class Settings(BaseSettings):
     DEBUG: bool = Field(default=False)
     SECRET_KEY: str = Field(...)
     API_VERSION: str = Field(default="v1")
+    APP_VERSION: str = Field(default="1.6.0")
     
     # ===================================
     # DATABASE