chore(deps): keep next on 16.3.8, 16.4.0 breaks history-driven URL state

With 16.4.0 a document sheet opened through ?doc= stays open after Back,
and the [ / ] day keys sometimes leave ?day= where it was. Three end-to-end
journeys fail 3 of 40 repeated runs on 16.4.0 and 0 of 80 on 16.3.8 with
every other update in place, and CI hit the same journeys on both
viewports. 16.3.8 already carries every published security fix, so
Dependabot skips 16.4.0 for next and the two packages that move with it.

package.json

@@ -80,7 +80,7 @@
     "jspdf-autotable": "^5.0.8",
     "lucide-react": "^1.52.0",
     "mmdb-lib": "^3.0.3",
-    "next": "16.4.0",
+    "next": "16.3.8",
     "next-themes": "^0.4.6",
     "nodemailer": "10.0.15",
     "otpauth": "^9.5.2",