Add root .prettierrc pinning prettier settings (closes #197)

Add a root `.prettierrc` (JSON) with `tabWidth: 4` and
`proseWrap: always`, the shared house style, so prettier formatting no
longer depends on host prettier defaults. Drop the now-redundant inline
`--tab-width 4` from `script/fmt` so the config file is the single
source of truth; `script/fmt` still formats only `static/js/*.js` and
`.prettierignore` still keeps the vendored `alpine.min.js` out.

Verified `make fmt` on a clean tree is a no-op: all first-party JS
reports unchanged and the vendored bundle is untouched.

Model: opus-4-8

script/fmt

@@ -8,7 +8,7 @@ main() {
     cd "$ROOT"
     gofmt -s -w .
     goimports -w .
-    npx prettier --write --tab-width 4 static/js/*.js
+    npx prettier --write static/js/*.js
 }
 
 main "$@"