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/models/app_settings.rs

@@ -72,6 +72,8 @@ pub struct AppSettings {
     pub timezone: Option<String>,
     #[serde(default)]
     pub default_microphone: Option<String>,
+    #[serde(default)]
+    pub dismissed_image_digest: Option<String>,
 }
 
 impl Default for AppSettings {
@@ -90,6 +92,7 @@ impl Default for AppSettings {
             dismissed_update_version: None,
             timezone: None,
             default_microphone: None,
+            dismissed_image_digest: None,
         }
     }
 }