chore: pin prettier to 3.9.5 and run format check after install (#1716)

The "Check formatting" step in main.yml ran `npx prettier --check .`
before `npm install`, so npx fetched the LATEST prettier rather than the
version pinned in package-lock.json. This made the format check
non-deterministic and red on open PRs whenever the latest published
prettier differed from the pinned one.

- Pin prettier to exact 3.9.5 in devDependencies and update the lockfile
- Reformat the repo with the pinned prettier (expected source churn)
- Move the format check after `npm install --no-package-lock` and run it
  via `npm run prettier-check` so CI uses the locally-installed pinned
  prettier


Claude-Session: https://claude.ai/code/session_01HiccCEh9mwCfVzE8qYcop1

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/src/components/DynamicJsonForm.tsx

@@ -347,8 +347,7 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
           // Titled single-select using oneOf/anyOf with const/title pairs
           const titledOptions = (
             (propSchema.oneOf ?? propSchema.anyOf) as
-              | (JsonSchemaType | JsonSchemaConst)[]
-              | undefined
+              (JsonSchemaType | JsonSchemaConst)[] | undefined
           )?.filter((opt): opt is JsonSchemaConst => "const" in opt);
 
           if (titledOptions && titledOptions.length > 0) {
@@ -606,8 +605,7 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
 
           const titledMulti = (
             (itemSchema.anyOf ?? itemSchema.oneOf) as
-              | (JsonSchemaType | JsonSchemaConst)[]
-              | undefined
+              (JsonSchemaType | JsonSchemaConst)[] | undefined
           )?.filter((opt): opt is JsonSchemaConst => "const" in opt);
 
           if (titledMulti && titledMulti.length > 0) {