[FEATURE] Add support for automatic theme switching via system color-scheme preference

[vhsdream]

Proposed changes

• Uses the value of PVEThemeCookie to determine selected theme rather than the presence of theme-proxmox-dark.css.
• Allows for automatic theme switch between light + dark modes when Default (Auto) is selected as the theme.
• Checks for the prefers-color-scheme: dark attribute, which is how Proxmox itself determines when to use light or dark modes.

Notes

• Tested in Firefox and Chromium on Linux (system pref managed with /org/gnome/desktop/interface/color-scheme 'prefer-light/prefer-dark').
• Tested in Firefox, Edge and Chrome on Windows via Settings > Personalization > Themes.
• I don't have MacOS virtualized so can't test if that uses the same method of setting light/dark mode preference.
• I also am very new to working with javascript and CSS, so I'm not sure if the method I used in index.html.tpl is the best way to achieve this.