chore: migrate to Yarn Berry (4.16.0)

Adds .yarnrc.yml and pins packageManager in package.json so installs
use yarn 4 with the node-modules linker instead of yarn classic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.yarnrc.yml

@@ -0,0 +1,8 @@
+approvedGitRepositories:
+  - "**"
+
+enableScripts: true
+
+nodeLinker: node-modules
+
+npmMinimalAgeGate: 0

package.json

@@ -40,5 +40,6 @@
     "typescript": "^5.9.2",
     "vite": "^7.1.7",
     "vite-tsconfig-paths": "^5.1.4"
-  }
+  },
+  "packageManager": "yarn@4.16.0+sha512.5374c94eb4ef6aa8188fb112f20c1aa6569f248d676c5e576e1fd2a1a4d8d87a96df65d9dfe1c2a0252cbe38bda46cf18d955005b81b43cc7607a5c9d56fd2b6"
 }