build: use latest pnpm (12.x) managed by mise

- mise.toml: pnpm = latest
- pnpm 12 approves build scripts via allowBuilds in pnpm-workspace.yaml
- drop packageManager pin (mise is the version source; pnpm would
  self-downgrade to it) and the dead yarn pin in the root package.json

mta-sign-ui/package.json

@@ -36,13 +36,5 @@
     "jsdom": "^28.0.0",
     "typescript": "^5.7.3",
     "vitest": "^4.0.0"
-  },
-  "packageManager": "pnpm@10.33.0",
-  "pnpm": {
-    "onlyBuiltDependencies": [
-      "esbuild",
-      "sharp",
-      "unrs-resolver"
-    ]
   }
 }

mta-sign-ui/pnpm-workspace.yaml

@@ -0,0 +1,4 @@
+allowBuilds:
+  esbuild: true
+  sharp: true
+  unrs-resolver: true