feat: module-driven onboarding system + simplified 3-step signup

Add OnboardingProviderProtocol so modules declare their own post-signup
onboarding steps. The core OnboardingAggregator discovers enabled
providers and exposes a dashboard API (GET /dashboard/onboarding).
A session-scoped banner on the store dashboard shows a checklist that
guides merchants through setup without blocking signup.

Signup is simplified from 4 steps to 3 (Plan → Account → Payment):
store creation is merged into account creation, store language is
captured from the user's browsing language, and platform-specific
template branching is removed.

Includes 47 unit and integration tests covering all new providers,
the aggregator, the API endpoint, and the signup service changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/core/static/store/js/dashboard.js

@@ -10,6 +10,47 @@ const storeDashLog = window.LogConfig.loggers.dashboard ||
 storeDashLog.info('Loading...');
 storeDashLog.info('[STORE DASHBOARD] data function exists?', typeof data);
 
+/**
+ * Onboarding banner component.
+ * Fetches onboarding steps from API, supports session-scoped dismiss.
+ */
+function onboardingBanner() {
+    return {
+        visible: false,
+        steps: [],
+        totalSteps: 0,
+        completedSteps: 0,
+        progressPercentage: 0,
+
+        async init() {
+            // Check session-scoped dismiss
+            if (sessionStorage.getItem('onboarding_dismissed')) {
+                return;
+            }
+
+            try {
+                const response = await apiClient.get('/store/dashboard/onboarding');
+                this.steps = response.steps || [];
+                this.totalSteps = response.total_steps || 0;
+                this.completedSteps = response.completed_steps || 0;
+                this.progressPercentage = response.progress_percentage || 0;
+
+                // Show banner only if there are incomplete steps
+                if (this.totalSteps > 0 && !response.all_completed) {
+                    this.visible = true;
+                }
+            } catch (error) {
+                storeDashLog.error('Failed to load onboarding status', error);
+            }
+        },
+
+        dismiss() {
+            sessionStorage.setItem('onboarding_dismissed', 'true');
+            this.visible = false;
+        }
+    };
+}
+
 function storeDashboard() {
     storeDashLog.info('[STORE DASHBOARD] storeDashboard() called');
     storeDashLog.info('[STORE DASHBOARD] data function exists inside?', typeof data);