fix: resolve settings page icon and 404 errors

- Change icon from 'envelope' to 'mail' (envelope not in icons.js)
- Add default query param to GET /admin/settings/{key} endpoint
- Return AdminSettingDefaultResponse instead of 404 when default provided
- Update loadShippingSettings() to use default param for carrier settings

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

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

static/admin/js/settings.js

@@ -242,24 +242,24 @@ function adminSettings() {
 
         async loadShippingSettings() {
             try {
-                // Load each carrier setting
-                const carriers = ['greco', 'colissimo', 'xpresslogistics'];
+                // Load each carrier setting with defaults to avoid 404 errors
+                const carriers = [
+                    { name: 'greco', default: 'https://dispatchweb.fr/Tracky/Home/' },
+                    { name: 'colissimo', default: '' },
+                    { name: 'xpresslogistics', default: '' }
+                ];
                 for (const carrier of carriers) {
-                    try {
-                        const key = `carrier_${carrier}_label_url`;
-                        const data = await apiClient.get(`/admin/settings/${key}`);
-                        if (data && data.value) {
-                            this.shippingSettings[key] = data.value;
-                        }
-                    } catch (error) {
-                        // Setting doesn't exist yet, use default
-                        settingsLog.debug(`Setting carrier_${carrier}_label_url not found, using default`);
+                    const key = `carrier_${carrier.name}_label_url`;
+                    // Use default query param to avoid 404 for non-existent settings
+                    const data = await apiClient.get(`/admin/settings/${key}?default=${encodeURIComponent(carrier.default)}`);
+                    if (data && data.value !== undefined) {
+                        this.shippingSettings[key] = data.value;
                     }
                 }
                 settingsLog.info('Shipping settings loaded:', this.shippingSettings);
             } catch (error) {
                 settingsLog.error('Failed to load shipping settings:', error);
-                // Don't show error for missing settings, just use defaults
+                // On error, keep existing defaults
             }
         },