refactor: move campaign editing from modal to separate page

- Create AdminCampaignEdit page for editing campaigns
- Remove CampaignModal from AdminCampaigns
- Use centralized icons (CampaignIcon, ChartIcon added)
- Add route /admin/campaigns/:id/edit
- Add localization keys for loadError and updateError

src/locales/en.json

@@ -1489,7 +1489,9 @@
         "deleteText": "This action cannot be undone. Campaigns with registrations cannot be deleted.",
         "cancel": "Cancel",
         "delete": "Delete"
-      }
+      },
+      "loadError": "Failed to load campaign",
+      "updateError": "Failed to save changes"
     },
     "promocodes": {
       "title": "Promo Codes",