Add light/dark mode toggle

[ssrahul96]

Fixed https://github.com/lyqht/mini-qr/issues/61

Added toggle button next to Github link

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Name	Status	Preview	Comments	Updated (UTC)
mini-qr-code-generator	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 29, 2024 3:54pm

[lyqht]

Hi @ssrahul96, thanks for the PR! I've refactored it so that

• there will be a 3rd toggle for system-preference
• dark mode button should still be using the app's default color
• the whole button should be a button element for accessibility rather than an anchor element like github.