chore: remove the sponsor material for Doubao and Atlas Cloud

The about pages lose the "Sponsored by ByteDance Doubao" card, the
promotional links in the provider list and the thanks paragraph; the
"bring your own key" note stays. The quota toast no longer shows the
Volcengine sign-up paragraph, and its messages in the four languages no
longer mention a sponsor. The provider guides lose the "free tokens"
promo line. The logo and invite images used only by that material are
deleted. Doubao and Atlas Cloud remain supported providers.

components/quota-limit-toast.tsx

@@ -85,18 +85,6 @@ export function QuotaLimitToast({
             {/* Message */}
             <div className="text-sm text-muted-foreground leading-relaxed mb-4 space-y-2">
                 <p>{quotaMessage}</p>
-                {!isSelfHosted && (
-                    <p
-                        dangerouslySetInnerHTML={{
-                            __html: formatMessage(
-                                dict.quota.doubaoSponsorship,
-                                {
-                                    link: "https://www.volcengine.com/activity/codingplan?ac=MMAP8JTTCAQ2&rc=Z9Z3LDTJ&utm_campaign=drawio&utm_content=drawio&utm_medium=devrel&utm_source=OWO&utm_term=drawio",
-                                },
-                            ),
-                        }}
-                    />
-                )}
                 <p
                     dangerouslySetInnerHTML={{
                         __html: tipHtml,