Add explicit browserslist to frontend package.json

Ensures deterministic CSS output across build environments (local vs
Docker/Alpine) by pinning browser targets.

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

frontend/package.json

@@ -53,6 +53,12 @@
     "vitest": "^4.0.18",
     "vue-tsc": "^3.2.5"
   },
+  "browserslist": [
+    ">= 0.5%",
+    "last 2 versions",
+    "Firefox ESR",
+    "not dead"
+  ],
   "engines": {
     "node": "^20.19.0 || >=22.12.0"
   }