feat: UI polish — AI enhancements branding, footer year, admin background fix

- Header: add "+ AI enhancements" italic subtitle below logo
- Footer: update copyright year to 2021–current, add "AI enhancements by Cloud Host" link
- Admin page: wrap all states in pageBg Box so dark-mode background shows correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pages/admin/index.tsx

@@ -46,6 +46,7 @@ const AdminPage: NextPage = () => {
     const [testResult, setTestResult] = useState<string | null>(null);
     const [testLoading, setTestLoading] = useState(false);
 
+    const pageBg = useColorModeValue("gray.50", "gray.900");
     const cardBg = useColorModeValue("white", "gray.800");
     const borderCol = useColorModeValue("gray.200", "gray.600");
     const codeBg = useColorModeValue("gray.100", "gray.700");
@@ -140,7 +141,7 @@ const AdminPage: NextPage = () => {
 
     if (authed === null) {
         return (
-            <Box w="full" display="flex" justifyContent="center" pt={20}>
+            <Box w="full" minH="100%" bg={pageBg} display="flex" justifyContent="center" pt={20}>
                 <Spinner size="xl" color="lingva.400" />
             </Box>
         );
@@ -150,6 +151,7 @@ const AdminPage: NextPage = () => {
         return (
             <>
                 <CustomHead home={false} />
+                <Box w="full" minH="100%" bg={pageBg}>
                 <Box
                     w="full" maxW="400px" mx="auto" mt={10} p={8}
                     bg={cardBg} borderWidth={1} borderColor={borderCol}
@@ -181,6 +183,7 @@ const AdminPage: NextPage = () => {
                         </NextLink>
                     </VStack>
                 </Box>
+                </Box>
             </>
         );
     }
@@ -188,7 +191,8 @@ const AdminPage: NextPage = () => {
     return (
         <>
             <CustomHead home={false} />
-            <Box w="full" maxW="700px" mx="auto" px={4} pb={10}>
+            <Box w="full" minH="100%" bg={pageBg}>
+            <Box w="full" maxW="700px" mx="auto" px={4} pb={10} pt={2}>
                 <HStack justify="space-between" mb={6}>
                     <NextLink href="/" passHref>
                         <Button as={Link} leftIcon={<FiArrowLeft />} variant="ghost" size="sm">Back</Button>
@@ -384,6 +388,7 @@ const AdminPage: NextPage = () => {
                     </Button>
                 </VStack>
             </Box>
+            </Box>
         </>
     );
 };