New design language:
- dark background, system sans for UI, monospace for data
- single green accent, amber/red for warn/critical
- square-bordered panels + tables, no rounded cards or shadows
- status conveyed via left-border on overview cards + badges
Changes:
- new app.css defines CSS vars + component classes (.panel, .tbl,
.card, .btn, .input, .badge with [data-status=*])
- new ServerWeb.DashboardNav function component for a shared top nav
with active-link highlighting; replaces per-view navigation clutter
- strip the Phoenix welcome scaffold (logo, version badge, twitter/GH
links) from layouts/app.html.heex; leaves only flash + content
- root.html.heex title suffix switched to 'Proxmox Monitor', body
loses the Tailwind-white background
- rewrite render/1 in all four LiveViews + login template to use the
new classes; admin form now uses <.form for={@form}> and properly
clears on success
- login page redesigned to a single tight panel matching the rest
All 58 tests still pass; 'mix compile --warnings-as-errors' is clean.
