fix(admin): dark background on outline buttons (Users page)

Root cause: the shadcn outline Button variant uses bg-background,
which resolves to the light-theme CSS variable since the admin panel
doesn't wrap itself in a .dark class — so every outline button here
had a white background regardless of border/text color overrides,
making the colored text glare instead of read cleanly. Explicit
bg-zinc-900/hover:bg-zinc-800 on all five outline buttons on this
page (Message, Export, Warn, Un-shadowban, Unban) fixes it.

src/app/p/[secret]/users/page.tsx

@@ -282,7 +282,7 @@ export default function AdminUsersPage() {
                   <Button
                     size="sm"
                     variant="outline"
-                    className="border-sky-800 text-sky-300 hover:text-sky-100"
+                    className="bg-zinc-900 border-sky-800 text-sky-300 hover:bg-zinc-800 hover:text-sky-100"
                     onClick={() => startConversationMutation.mutate({ targetPetId: user.pets[0].id })}
                     disabled={startConversationMutation.isPending}
                     title={`Message ${user.pets[0].name} as PawFeed Team`}
@@ -299,7 +299,7 @@ export default function AdminUsersPage() {
                 <Button
                   size="sm"
                   variant="outline"
-                  className="border-blue-800 text-blue-300 hover:text-blue-100"
+                  className="bg-zinc-900 border-blue-800 text-blue-300 hover:bg-zinc-800 hover:text-blue-100"
                   onClick={() => exportDataMutation.mutate({ ownerId: user.id })}
                   disabled={exportDataMutation.isPending}
                   title="Datenauskunft (Art. 15 DSGVO) per E-Mail versenden"
@@ -314,7 +314,7 @@ export default function AdminUsersPage() {
                 <Button
                   size="sm"
                   variant="outline"
-                  className="border-amber-800 text-amber-300 hover:text-amber-100"
+                  className="bg-zinc-900 border-amber-800 text-amber-300 hover:bg-zinc-800 hover:text-amber-100"
                   onClick={() => setWarnTarget(user.id)}
                   title="Warnings"
                 >
@@ -336,7 +336,7 @@ export default function AdminUsersPage() {
                   <Button
                     size="sm"
                     variant="outline"
-                    className="border-violet-800 text-violet-300 hover:text-violet-100"
+                    className="bg-zinc-900 border-violet-800 text-violet-300 hover:bg-zinc-800 hover:text-violet-100"
                     onClick={() => setUnshadowbanTarget(user.id)}
                     title="Remove shadowban"
                   >
@@ -359,7 +359,7 @@ export default function AdminUsersPage() {
                   <Button
                     size="sm"
                     variant="outline"
-                    className="border-zinc-700 text-zinc-300 hover:text-zinc-100"
+                    className="bg-zinc-900 border-zinc-700 text-zinc-300 hover:bg-zinc-800 hover:text-zinc-100"
                     onClick={() => setUnbanTarget(user.id)}
                   >
                     <ShieldCheck className="h-4 w-4 mr-1.5" />