fix: resolve architecture validation warnings

- Replace window.apiClient with apiClient in feature-store.js (JS-002)
- Replace window.apiClient with apiClient in upgrade-prompts.js (JS-002)
- Replace inline SVGs with $icon() helper in features.html (FE-002)
- Add check-circle-filled icon to icons.js

Architecture validation now passes with 0 errors, 0 warnings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/shared/js/feature-store.js

@@ -80,7 +80,7 @@
                 this.error = null;
 
                 // Fetch available features (lightweight endpoint)
-                const response = await window.apiClient.get('/vendor/features/available');
+                const response = await apiClient.get('/vendor/features/available');
 
                 this.features = response.features || [];
                 this.tierCode = response.tier_code;
@@ -108,7 +108,7 @@
             if (!vendorCode) return;
 
             try {
-                const response = await window.apiClient.get('/vendor/features');
+                const response = await apiClient.get('/vendor/features');
 
                 // Build map for quick lookup
                 this.featuresMap = {};