fix(provider-form): show Codex FAST mode toggle in account-select mode

The FAST mode switch was gated on mode === "manage", but the Claude and
Claude Desktop Codex OAuth forms render CodexOAuthSection in select mode
while still passing onFastModeChange. The toggle was therefore hidden:
new providers couldn't enable FAST mode and existing ones couldn't turn
it off, even though the backend still honors meta.codexFastMode. Gate the
toggle on onFastModeChange presence instead, restoring the pre-refactor
behavior without affecting the manage panel or native Codex form (which
do not pass the handler).

src/components/providers/forms/CodexOAuthSection.tsx

@@ -198,7 +198,7 @@ export const CodexOAuthSection: React.FC<CodexOAuthSectionProps> = ({
         accountSelect
       )}
 
-      {mode === "manage" && onFastModeChange && (
+      {onFastModeChange && (
         <div className="flex items-center justify-between rounded-md border bg-muted/30 p-3">
           <div className="space-y-1 pr-4">
             <Label className="text-sm font-medium">