chore(deps): bump react-i18next to ^15.7.4

The 15.5.0 pin existed only to avoid react-i18next's `typescript: ^5`
peer while we were on TS 4.9.5. Now that the upgrade moved us to TS 5.9,
that constraint is gone, so move to the latest 15.x (caret) for fixes.

Stays within i18next 23 (15.7.4 peers i18next >= 23.4.0); going to
react-i18next 16/17 would require an i18next major bump (25/26), which
is a separate upgrade and intentionally out of scope here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Riippi

package.json

@@ -40,7 +40,7 @@
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-hook-form": "^7.76.1",
-    "react-i18next": "15.5.0",
+    "react-i18next": "^15.7.4",
     "react-router-dom": "^6.30.3",
     "sass": "^1.77.2",
     "typescript": "5.9.3",