refactor(ui): minimalistic utilitarian redesign across all views

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.

server/lib/server_web/components/layouts/app.html.heex

@@ -1,32 +1,7 @@
-<header class="px-4 sm:px-6 lg:px-8">
-  <div class="flex items-center justify-between border-b border-zinc-100 py-3 text-sm">
-    <div class="flex items-center gap-4">
-      <a href="/">
-        <img src={~p"/images/logo.svg"} width="36" />
-      </a>
-      <p class="bg-brand/5 text-brand rounded-full px-2 font-medium leading-6">
-        v<%= Application.spec(:phoenix, :vsn) %>
-      </p>
-    </div>
-    <div class="flex items-center gap-4 font-semibold leading-6 text-zinc-900">
-      <a href="https://twitter.com/elixirphoenix" class="hover:text-zinc-700">
-        @elixirphoenix
-      </a>
-      <a href="https://github.com/phoenixframework/phoenix" class="hover:text-zinc-700">
-        GitHub
-      </a>
-      <a
-        href="https://hexdocs.pm/phoenix/overview.html"
-        class="rounded-lg bg-zinc-100 px-2 py-1 hover:bg-zinc-200/80"
-      >
-        Get Started <span aria-hidden="true">&rarr;</span>
-      </a>
-    </div>
-  </div>
-</header>
-<main class="px-4 py-20 sm:px-6 lg:px-8">
-  <div class="mx-auto max-w-2xl">
-    <.flash_group flash={@flash} />
-    <%= @inner_content %>
-  </div>
-</main>
+<%= if info = Phoenix.Flash.get(@flash, :info) do %>
+  <div class="flash info">{info}</div>
+<% end %>
+<%= if err = Phoenix.Flash.get(@flash, :error) do %>
+  <div class="flash err">{err}</div>
+<% end %>
+{@inner_content}