style(ui): Polish the UI

[jasoryeh]

Hello,

I discovered Kopia a bit ago, but the one thing that made this tool a bit intimidating was the user interface.

Here are a few stylistic changes to polish up some interfaces, still working on adding some other adjustments here and there to make the UI more friendly.

Among these changes:

• Spacings between things like form items and elements to make everything less "squished" together
• Fix dark mode inconsistencies (snapshot table)
• De-clutter the policy editor to not be one giant accordion
• Add some text to make the purpose of each page clearer
• Moving the connected repository name to the navbar, and putting it at the top right
• and more

Please let me know if there are any changes (to this PR or new changes) on your mind!