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/upgrade-prompts.js

@@ -77,7 +77,7 @@
                 this.loading = true;
                 this.error = null;
 
-                const response = await window.apiClient.get('/vendor/usage');
+                const response = await apiClient.get('/vendor/usage');
                 this.usage = response;
                 this.loaded = true;
 
@@ -158,7 +158,7 @@
          */
         async checkLimitAndProceed(limitType, onSuccess) {
             try {
-                const response = await window.apiClient.get(`/vendor/usage/check/${limitType}`);
+                const response = await apiClient.get(`/vendor/usage/check/${limitType}`);
 
                 if (response.can_proceed) {
                     if (typeof onSuccess === 'function') {