fix: make auth canvas scroll actually constrain on short viewports (#1523)

PR #1521 added max-height and overflow-y-auto but the card stayed content-
sized because flex items default to min-height:auto and justify-center
clipped overflow while html/body stay overflow:hidden. Add min-h-0 and
my-auto, use dvh, scroll the auth shell, and pin Enter Sencho outside the
preflight scroll region.

frontend/src/components/Setup.tsx

@@ -93,9 +93,18 @@ export function Setup({ onComplete, className, ...props }: SetupProps & React.Co
       <div className={cn('relative', className)} {...props}>
         <AuthCanvas
           footer={
-            <div className="flex items-center justify-between">
-              <span>Console · First boot</span>
-              <span className="text-stat-subtitle/70">Account ready</span>
+            <div className="flex flex-col gap-4">
+              <Button
+                type="button"
+                onClick={onComplete}
+                className="h-11 w-full bg-brand text-brand-foreground shadow-btn-glow hover:bg-brand/90"
+              >
+                Enter Sencho<ArrowRight strokeWidth={1.5} />
+              </Button>
+              <div className="flex items-center justify-between border-t border-card-border/60 pt-3">
+                <span>Console · First boot</span>
+                <span className="text-stat-subtitle/70">Account ready</span>
+              </div>
             </div>
           }
         >
@@ -106,13 +115,6 @@ export function Setup({ onComplete, className, ...props }: SetupProps & React.Co
               caption="A quick check that this host can run Docker deploys. Warnings won't stop you; each one carries a fix."
             />
             <EnvironmentChecks />
-            <Button
-              type="button"
-              onClick={onComplete}
-              className="h-11 w-full bg-brand text-brand-foreground shadow-btn-glow hover:bg-brand/90"
-            >
-              Enter Sencho<ArrowRight strokeWidth={1.5} />
-            </Button>
           </div>
         </AuthCanvas>
       </div>