feat: modernize login page UI and add password toggle

[maks-oleksyuk]

I've been working a bit on the admin panel layout, and I thought it could look better, specifically:

• using Bootstrap classes
• updating to a minimalist design (looking forward to feedback)
• fixing bugs (the form wasn’t centered correctly relative to the page when the profiler was active, and the title alignment was off)
• adding password preview functionality
• adding dark theme support

I’d appreciate any reviews or feedback on this

Here are a few screenshots of how it looked before and after