Rename AuthMode to Backend, fix LiteLLM variant typo, add image update alerts, clean up Settings

- Fix serde deserialization error: TypeScript sent "lit_llm" but Rust expected "lite_llm"
- Rename AuthMode enum to Backend across Rust and TypeScript (with serde alias for backward compat)
- Add container image update checking via registry digest comparison
- Improve Settings page: fix image address display spacing, remove per-project auth section
- Update UI labels from "Auth" to "Backend" throughout

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

TECHNICAL.md

@@ -290,7 +290,7 @@ triple-c/
             │   ├── image.rs       # Build from Dockerfile, pull from registry
             │   └── network.rs     # Per-project bridge networks for MCP
             ├── models/            # Data structures
-            │   ├── project.rs     # Project, AuthMode, BedrockConfig
+            │   ├── project.rs     # Project, Backend, BedrockConfig
             │   ├── mcp_server.rs  # MCP server configuration
             │   ├── app_settings.rs # Global settings (image source, AWS, etc.)
             │   ├── container_config.rs # Image name resolution