feat(admin): implement navigation & layout (Plan 02, Phases 1–3)

Phase 1 — Active route highlighting
- NavMain derives isActive from usePathname() at render time
- Flat links: exact match; collapsible groups: startsWith; sub-items: exact match
- Removed hardcoded isActive fields from NAV_LINKS

Phase 2 — Dynamic breadcrumbs
- Added ROUTE_LABELS map to app.constants.ts
- Created DynamicBreadcrumb component (pathname-driven, ShadCN primitives)
- Wired DynamicBreadcrumb into dashboard layout header

Phase 3 — Mobile shell (observation only, no structural changes needed)

Icons — migrated all sidebar/breadcrumb icons from lucide-react to @hugeicons/react + @hugeicons/core-free-icons

Infra
- tsconfig.json: moduleResolution → bundler (required for hugeicons ESM exports)
- Renamed middleware.ts → proxy.ts (Next.js 15.5 convention)
- Added @hugeicons/mcp-server to .mcp.json

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -24,7 +24,7 @@
     "@clerk/backend": "^2.32.1",
     "@tailwindcss/postcss": "^4.2.0",
     "convex": "^1.32.0",
-    "next": "^15.3.2",
+    "next": "16.1.6",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "stripe": "^20.4.0",