fix(blazor): pin Tailwind CSS to v3 to restore CI build (#142)

Tailwind CSS v4 removed the CLI from the main package and changed
@apply semantics (custom component classes can no longer be cross-
referenced via @apply). Since the Blazor demo app's CSS extensively
uses this pattern (~20 component classes), pin to v3 for now.

A full v4 migration should be tracked as a separate issue.

Author: phmatray