Add global backend defaults with runtime fallback

New fields: GlobalAwsSettings.default_model_id, plus
GlobalOllamaSettings and GlobalOpenAiCompatibleSettings (base_url +
default_model_id each). When a per-project base_url or model_id is
blank, the container env vars and config fingerprints fall back to
the global value. Container recreation is triggered whenever the
resolved value changes, so editing a global default updates existing
projects on next start.

UI: added the new fields to AwsSettings and two new global settings
components, slotted into the Backends accordion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/components/settings/AwsSettings.tsx

@@ -12,6 +12,7 @@ export default function AwsSettings() {
     aws_config_path: null,
     aws_profile: null,
     aws_region: null,
+    default_model_id: null,
   };
 
   // Load profiles when component mounts or aws_config_path changes
@@ -105,6 +106,18 @@ export default function AwsSettings() {
             className="w-full px-2 py-1.5 text-xs bg-[var(--bg-primary)] border border-[var(--border-color)] rounded focus:outline-none focus:border-[var(--accent)]"
           />
         </div>
+
+        {/* Default Model ID */}
+        <div>
+          <span className="text-[var(--text-secondary)] text-xs block mb-1">Default Model ID<Tooltip text="Default Bedrock model ID. Used when a Bedrock project doesn't set its own Model ID." /></span>
+          <input
+            type="text"
+            value={globalAws.default_model_id ?? ""}
+            onChange={(e) => handleChange("default_model_id", e.target.value)}
+            placeholder="anthropic.claude-sonnet-4-20250514-v1:0"
+            className="w-full px-2 py-1.5 text-xs bg-[var(--bg-primary)] border border-[var(--border-color)] rounded focus:outline-none focus:border-[var(--accent)]"
+          />
+        </div>
       </div>
     </div>
   );