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>

app/src-tauri/src/commands/terminal_commands.rs

@@ -1,6 +1,6 @@
 use tauri::{AppHandle, Emitter, State};
 
-use crate::models::{AuthMode, BedrockAuthMethod, Project};
+use crate::models::{Backend, BedrockAuthMethod, Project};
 use crate::AppState;
 
 /// Build the command to run in the container terminal.
@@ -9,7 +9,7 @@ use crate::AppState;
 /// the AWS session first. If the SSO session is expired, runs `aws sso login`
 /// so the user can re-authenticate (the URL is clickable via xterm.js WebLinksAddon).
 fn build_terminal_cmd(project: &Project, state: &AppState) -> Vec<String> {
-    let is_bedrock_profile = project.auth_mode == AuthMode::Bedrock
+    let is_bedrock_profile = project.backend == Backend::Bedrock
         && project
             .bedrock_config
             .as_ref()