feat: add OpenClaw User-Agent toggle, default off

Add a switch in the OpenClaw provider form to optionally send a browser
User-Agent header. The toggle defaults to off — only providers that
explicitly include headers in their preset or config will have it enabled.

Remove the previous auto-injection logic that force-added User-Agent on
every preset load and new provider creation.

src/components/providers/forms/ProviderForm.tsx

@@ -1450,6 +1450,8 @@ export function ProviderForm({
             onApiChange={openclawForm.handleOpenclawApiChange}
             models={openclawForm.openclawModels}
             onModelsChange={openclawForm.handleOpenclawModelsChange}
+            userAgent={openclawForm.openclawUserAgent}
+            onUserAgentChange={openclawForm.handleOpenclawUserAgentChange}
           />
         )}