feat(FN-2893): add merge-description disclosures in settings

- Render merge strategy descriptions behind expandable disclosure sections in Settings modal
- Add dedicated disclosure styling and update task form expectations for the new copy structure
- Extend Settings modal tests to cover disclosure toggling and merge-description visibility
- Improve model onboarding API key action layout on mobile by stretching form width and aligning button placement

packages/dashboard/app/components/SettingsModal.css

@@ -622,6 +622,30 @@
   content: "▾";
 }
 
+.settings-option-details {
+  margin-top: var(--space-xs);
+}
+
+.settings-option-details > summary {
+  cursor: pointer;
+  color: var(--text-muted);
+  font-size: calc(var(--space-sm) + var(--space-xs));
+  list-style: none;
+}
+
+.settings-option-details > summary::-webkit-details-marker {
+  display: none;
+}
+
+.settings-option-details > summary::before {
+  content: "▸";
+  margin-right: var(--space-xs);
+}
+
+.settings-option-details[open] > summary::before {
+  content: "▾";
+}
+
 .remote-tunnel-actions {
   margin-top: var(--space-md);
 }