refactor: derive admin registry from PROVIDER_INFO, simplify page state

- Provider options, labels, and base-URL placeholders now come from
  PROVIDER_INFO instead of hand-copied lists (fixes SiliconFlow .com/.cn
  placeholder drift; panel names now match the model-config dialog)
- Replace free-text subgroup strings + SUBGROUP_PROVIDERS reverse map
  with a typed provider field on SettingDef
- Precompute SETTINGS_BY_GROUP and PROVIDER_SUBGROUPS at module level
- Merge justSaved into saveMessage, drop unused mainRef, hoist
  fetchSettings out of the component, dedupe savedText logic
- Serialize from SETTINGS_REGISTRY directly; json validators in a map
  instead of a hardcoded key check
- Make allowPrivateUrls a function so ALLOW_PRIVATE_URLS edits in the
  admin panel apply without restart

app/api/validate-model/route.ts

@@ -51,7 +51,7 @@ export async function POST(req: Request) {
         }
 
         // SECURITY: Block SSRF attacks via custom baseUrl
-        if (baseUrl && !allowPrivateUrls && isPrivateUrl(baseUrl)) {
+        if (baseUrl && !allowPrivateUrls() && isPrivateUrl(baseUrl)) {
             return NextResponse.json(
                 { valid: false, error: "Invalid base URL" },
                 { status: 400 },