fix(admin): address Copilot review findings

- Reflect built-in defaults for boolean settings (ALLOW_PRIVATE_URLS
  defaults on) and allow clearing a saved boolean back to default,
  so the SSRF toggle matches actual runtime behavior.
- Harden JSON loading: filter settings values to strings only, and
  schema-validate stored ADMIN_PROVIDERS entries, dropping malformed
  ones instead of letting them reach runtime code.
- Set beforeunload returnValue so the unsaved-changes prompt shows in
  all browsers; reject non-finite numbers in settings validation.
- Fix README/CN/JA docs that claimed the panel auto-generates
  AI_MODELS_CONFIG (providers are merged at read time, not written).
- Add unit tests for corrupted-file value filtering and provider
  schema validation.

lib/admin/providers.ts

@@ -91,7 +91,13 @@ export function loadAdminProviders(): StoredAdminProvider[] {
     if (!raw) return []
     try {
         const parsed = JSON.parse(raw)
-        return Array.isArray(parsed) ? parsed : []
+        if (!Array.isArray(parsed)) return []
+        // Validate each entry's shape — a malformed/hand-edited value must
+        // not reach runtime code that assumes provider/models exist.
+        return parsed.flatMap((entry) => {
+            const result = AdminProviderSchema.safeParse(entry)
+            return result.success ? [result.data as StoredAdminProvider] : []
+        })
     } catch {
         console.error("[admin-providers] Failed to parse stored providers")
         return []