Consolidate all study-dependent routes and UI

- Remove global experiments and plugins routes; redirect to study-scoped
  pages
- Update sidebar navigation to separate platform-level and study-level
  items
- Add study filter to dashboard and stats queries
- Refactor participants, trials, analytics pages to use new header and
  breadcrumbs
- Update documentation for new route architecture and migration guide
- Remove duplicate experiment creation route
- Upgrade Next.js to 15.5.4 in package.json and bun.lock

package.json

@@ -62,7 +62,7 @@
     "date-fns": "^4.1.0",
     "drizzle-orm": "^0.41.0",
     "lucide-react": "^0.536.0",
-    "next": "^15.4.6",
+    "next": "^15.5.4",
     "next-auth": "^5.0.0-beta.29",
     "postgres": "^3.4.4",
     "react": "^19.0.0",