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/dashboard_nav.ex

@@ -0,0 +1,26 @@
+defmodule ServerWeb.DashboardNav do
+  @moduledoc "Top navigation bar rendered inside authenticated LiveViews."
+
+  use Phoenix.Component
+  use ServerWeb, :verified_routes
+
+  attr :active, :atom, required: true, values: [:overview, :vms, :admin, :other]
+
+  def nav(assigns) do
+    ~H"""
+    <nav class="nav">
+      <span class="brand">proxmox-monitor</span>
+      <span class="links">
+        <.link navigate={~p"/"} class={link_cls(@active, :overview)}>overview</.link>
+        <.link navigate={~p"/vms"} class={link_cls(@active, :vms)}>vms</.link>
+        <.link navigate={~p"/admin/hosts"} class={link_cls(@active, :admin)}>admin</.link>
+      </span>
+      <span class="spacer"></span>
+      <.link href={~p"/logout"} method="delete" class="signout">sign out</.link>
+    </nav>
+    """
+  end
+
+  defp link_cls(active, key) when active == key, do: "active"
+  defp link_cls(_, _), do: nil
+end