feat(web): login page with inline error handling

Add shadcn input/label/card primitives and implement the login page:
email/password form using useLogin, navigates to /objects on success,
shows inline i18n error on 401 (auth.invalid) or network failure.
2 new tests, 9 total green.

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

web/src/components/ui/label.tsx

@@ -0,0 +1,18 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+function Label({ className, ...props }: React.ComponentProps<"label">) {
+  return (
+    <label
+      data-slot="label"
+      className={cn(
+        "flex items-center gap-2 text-sm leading-none font-medium select-none group-data-[disabled=true]:pointer-events-none group-data-[disabled=true]:opacity-50 peer-disabled:cursor-not-allowed peer-disabled:opacity-50",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Label }