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/pnpm-workspace.yaml

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