fix: add null check for event.key in ThemeHotkey to prevent crash on browser autofill (fixes #10378)

[armorbreak001]

Background

The default theme-provider.tsx generated by npx shadcn@latest init contains a ThemeHotkey component that listens for the Ctrl+D keyboard shortcut to toggle the theme. The listener calls event.key.toLowerCase() without checking if event.key is defined.

During browser autofill/autocomplete on Chrome and Edge, the browser fires a synthetic keydown event where event.key is undefined. This causes an uncaught TypeError:

Uncaught TypeError: Cannot read properties of undefined (reading toLowerCase)
    at onKeyDown (theme-provider.tsx)

Solution

Add a null/undefined guard before accessing event.key.toLowerCase() in all affected files. This follows the same defensive pattern already used elsewhere in the handler (e.g., event.defaultPrevented check).

Changes

• templates/next-app/components/theme-provider.tsx: Added !event.key || guard
• templates/next-monorepo/apps/web/components/theme-provider.tsx: Added !event.key || guard
• templates/vite-app/src/components/theme-provider.tsx: Added !event.key || guard
• templates/vite-monorepo/apps/web/src/components/theme-provider.tsx: Added !event.key || guard
• apps/v4/app/(app)/create/lib/v0.ts: Added !event.key || guard

Verification

1. Create a new Next.js project with npx shadcn@latest init
2. Add <input type="email" /> to a page
3. Save an autofill entry in Chrome
4. Focus the input and select a suggestion from the browser autocomplete dropdown
5. Before fix: TypeError in console
6. After fix: Autofill completes normally, no errors

Closes #10378

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