feat: implement Session Mode UI for Fund Once, Ask Many interactions

- Fix slideStyles used-before-declaration error in onboarding.tsx that
  blocked typecheck (introduced by #79). Moved slideStyles const above
  the slides array that references it.
- Add missing DELETE /sessions/:id/history endpoint to sessions.ts.
  The frontend session.js calls this endpoint for the "Clear history"
  button but no matching route existed.

Fixes #67

artifacts/mobile/app/onboarding.tsx

@@ -21,6 +21,19 @@ import { ONBOARDING_COMPLETED_KEY } from "@/constants/storage-keys";
 const C = Colors.dark;
 const { width: SCREEN_WIDTH } = Dimensions.get("window");
 
+const slideStyles = StyleSheet.create({
+  iconCircle: {
+    width: 140,
+    height: 140,
+    borderRadius: 70,
+    backgroundColor: C.surfaceElevated,
+    borderWidth: 1,
+    borderColor: C.border,
+    alignItems: "center",
+    justifyContent: "center",
+  },
+});
+
 type Slide = {
   id: string;
   icon: React.ReactNode;
@@ -158,19 +171,6 @@ export default function OnboardingScreen() {
   );
 }
 
-const slideStyles = StyleSheet.create({
-  iconCircle: {
-    width: 140,
-    height: 140,
-    borderRadius: 70,
-    backgroundColor: C.surfaceElevated,
-    borderWidth: 1,
-    borderColor: C.border,
-    alignItems: "center",
-    justifyContent: "center",
-  },
-});
-
 const styles = StyleSheet.create({
   container: {
     flex: 1,