Ensure theme switches to preferred colour scheme on first load

[JakeYallop]

Description
Currently opening the page for the first time does not correctly apply the users preferred colour scheme.

In OptionsWaiter, we do this to apply the theme:

CyberChef/src/web/waiters/OptionsWaiter.mjs

Lines 177 to 184 in d735496

	applyPreferredColorScheme() {
	const themeFromStorage = this.app?.options?.theme;
	let theme = themeFromStorage;
	if (!theme) {
	theme = this.getPreferredColorScheme();
	}
	this.changeTheme(theme);
	}

However, themeFromStorage is already set to "classic" at this point, assigned here:

CyberChef/src/web/App.mjs

Line 36 in d735496

this.options = Object.assign({}, defaultOptions);

The fix is set the default theme to undefined and let it be set from either localStorage or getPreferredColorScheme().

Related issues: #1921, #1901

AI disclosure
No AI was used in the creation of this pull request.

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