Add dashboard loading states

[sanne-san]

Changes

• Add skeleton as loading state for initial page load.
• Add loading states for tab and datepicker transitions, where data is faded and pulses until new data is loaded.
• Speed up default Tailwind pulse animation.
• Add smooth bar width transitions for when switching between reports.

Dark mode

• The UI has been tested both in dark and light mode

[zoldar]

Really nice solution! It's way simpler than what I've anticipated. I've falsely assumed that patch application would look messy if transition effect didn't hide it. It's atomic enough to not cause cascades of reflows, cool. Also, awesome that phx navigation loading class could be reimplemented for full LV ✨ That will be a great help in other places too.

NOTE: As it's a PoC for now, I won't focus on minutiae of code organization or implementation details which are not too relevant at this stage. Whatever we come up with here will be rewritten anyway treating the current spike as a reference.

[zoldar]

Oh, that's nice 👍 Might help simplify loading state selectors I'm currently using in my branch.