feat(useFavicon): add temporary option to restore favicon on unmount

[udohjeremiah]

Feature Request: Support temporary favicon with cleanup on unmount

Current behavior

useFavicon sets the favicon but never restores the original when the component unmounts, making it impossible to use it temporarily on a specific page or component.

Proposed API

Add an optional options parameter with a temporary flag:

useFavicon(url, { temporary: true });

Notes on implementation

• options is stored in a ref (following the same pattern as useScript) so changes to the options object don't cause the effect to re-run unnecessarily.
• If no link[rel~="icon"] exists in the document (e.g. frameworks that only use a preload hint instead of an icon link), the hook creates one and removes it on cleanup rather than restoring an empty href. This handles environments like TanStack Start correctly.
• If the link already existed, the original href is restored as expected.

Use case

Setting a page-specific favicon that should revert to the app's default favicon when navigating away.

// Reverts to original favicon when component unmounts
useFavicon("https://example.com/favicon.ico", { temporary: true });