Update use-mobile hook for SSR compatibility

[christopherstyles]

When deploying with SSR using Kamal I was getting an error at the very end, on my healthcheck:

ERROR (Kamal::Cli::Healthcheck::Error): Exception while executing on host x.x.x.x: container not ready after 30 seconds (restarting)
Error: Process completed with exit code 1.

Logs¹ revealed the vite_ssr container crash-looping with ReferenceError: window is not defined. The root cause was the useIsMobile hook accessing window.matchMedia, so the SSR bundle crashed as soon as Node tried to import it.

The following changes were made:

• Removed all top-level window usage by lazily creating the MediaQueryList only when typeof window !== "undefined".
• Added a server snapshot to useSyncExternalStore so SSR renders a stable default (false).

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¹ bin/kamal app logs -r vite_ssr --since 10m