build(vite): inline purgecss plugin to drop vulnerable vite 6 dep

Replace the unmaintained vite-plugin-purgecss wrapper with a ~20-line
local plugin calling purgecss directly. The wrapper pinned vite@6.x,
which carried a high-severity dev-server WebSocket arbitrary file read
advisory (GHSA-p9ff-h696-f583) with no upstream fix. Build output and
behavior unchanged.

Author: HugoFara