Add SaaS frontend code (#5879)

# Description of Changes
Adds the code for the SaaS frontend as proprietary code to the OSS repo.
This version of the code is adapted from 22/1/2026, which was the last
SaaS version based on the 'V2' design. This will move us closer to being
able to have the OSS products understand whether the user has a SaaS
account, and provide the correct UI in those cases.

frontend/src/saas/components/onboarding/saasFlowResolver.ts

@@ -0,0 +1,40 @@
+import { TrialStatus } from '@app/auth/UseSession';
+import { FLOW_SEQUENCES, SlideId } from '@app/components/onboarding/saasOnboardingFlowConfig';
+
+export interface FlowConfig {
+  type: 'saas-trial' | 'saas-paid';
+  ids: SlideId[];
+}
+
+/**
+ * Resolves the appropriate onboarding flow based on user's subscription status.
+ *
+ * @param trialStatus - User's trial information from Supabase
+ * @param _isPro - Whether user has Pro subscription
+ * @returns FlowConfig with the appropriate slide sequence
+ */
+export function resolveSaasFlow(
+  trialStatus: TrialStatus | null,
+  _isPro: boolean | null
+): FlowConfig {
+  // Show free trial card if:
+  // 1. User has active trial (isTrialing = true)
+  // 2. Trial has not expired (daysRemaining > 0)
+  // 3. User is not paid Pro (or Pro is from trial)
+  const hasActiveTrial =
+    trialStatus?.isTrialing === true &&
+    trialStatus.daysRemaining > 0;
+
+  if (hasActiveTrial) {
+    return {
+      type: 'saas-trial',
+      ids: FLOW_SEQUENCES.saasTrialUser,
+    };
+  }
+
+  // For paid users, expired trials, or no trial info
+  return {
+    type: 'saas-paid',
+    ids: FLOW_SEQUENCES.saasPaidUser,
+  };
+}