chore(deps): pinia 2→3 + vue-router 4→5 (batch 4 phase A, #1449)

Both are Vue-3-compatible majors with no code impact for FC:
- vue-router 5: no breaking changes when not using file-based routing (FC uses a
  plain createRouter in router.js).
- pinia 3: drops Vue 2 + deprecated APIs; FC uses string-first setup-syntax
  defineStore + no custom pinia plugins, so nothing to change.
Phase A of the Vuetify-4 UI framework migration (milestone #158); Vuetify 4 lands
in phase B.

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

frontend/package.json

@@ -14,8 +14,8 @@
   },
   "dependencies": {
     "vue": "^3.4.0",
-    "vue-router": "^4.3.0",
-    "pinia": "^2.1.0",
+    "vue-router": "^5.0.0",
+    "pinia": "^3.0.0",
     "vuetify": "^3.5.0",
     "@mdi/font": "^7.4.0"
   },