refactor(ui): adopt Mantine Marquee and drop mount guards

Rebuild the integration marquee on Mantine's Marquee, SimpleGrid and
ThemeIcon instead of the hand-rolled grid, track and keyframes, and
reduce the stylesheet to the mask and reduced-motion rule.

Drop the useMounted guards from the login form and let the buttons rely
on the pending state alone, and use Group for the restore flow's file
summary.

apps/nextjs/src/components/backup/database-restore-flow.tsx

@@ -254,14 +254,14 @@ export const DatabaseRestoreFlow = ({ variant = "card", onRestoreComplete }: Dat
         <Group justify="space-between" wrap="nowrap">
           <Group gap="sm" wrap="nowrap" miw={0}>
             <IconFileZip size={20} />
-            <div style={{ minWidth: 0 }}>
+            <Group>
               <Text size="sm" fw={500} truncate>
                 {file?.name}
               </Text>
               <Text size="xs" c="dimmed" style={{ whiteSpace: "nowrap" }}>
                 {file ? `${(file.size / 1024 / 1024).toFixed(2)} MB` : ""}
               </Text>
-            </div>
+            </Group>
           </Group>
           <Button variant="subtle" size="xs" onClick={handleClear} style={{ flexShrink: 0 }}>
             {t("changeFile")}