Remove HA/Python components and add Phoenix LiveView dashboard

- Remove home-assistant-integration/ and dev-config/
- Remove Python/HA dependencies from flake.nix
- Add Phoenix LiveView dashboard with clean Elixir-only stack
- Add Node.js and PostgreSQL for Phoenix development
- Much cleaner monorepo architecture focusing on Elixir ecosystem

Next: Connect dashboard to MQTT for real-time host monitoring

server/dashboard/test/dashboard_web/controllers/error_html_test.exs

@@ -0,0 +1,14 @@
+defmodule DashboardWeb.ErrorHTMLTest do
+  use DashboardWeb.ConnCase, async: true
+
+  # Bring render_to_string/4 for testing custom views
+  import Phoenix.Template
+
+  test "renders 404.html" do
+    assert render_to_string(DashboardWeb.ErrorHTML, "404", "html", []) == "Not Found"
+  end
+
+  test "renders 500.html" do
+    assert render_to_string(DashboardWeb.ErrorHTML, "500", "html", []) == "Internal Server Error"
+  end
+end

server/dashboard/test/dashboard_web/controllers/error_json_test.exs

@@ -0,0 +1,12 @@
+defmodule DashboardWeb.ErrorJSONTest do
+  use DashboardWeb.ConnCase, async: true
+
+  test "renders 404" do
+    assert DashboardWeb.ErrorJSON.render("404.json", %{}) == %{errors: %{detail: "Not Found"}}
+  end
+
+  test "renders 500" do
+    assert DashboardWeb.ErrorJSON.render("500.json", %{}) ==
+             %{errors: %{detail: "Internal Server Error"}}
+  end
+end

server/dashboard/test/dashboard_web/controllers/page_controller_test.exs

@@ -0,0 +1,8 @@
+defmodule DashboardWeb.PageControllerTest do
+  use DashboardWeb.ConnCase
+
+  test "GET /", %{conn: conn} do
+    conn = get(conn, ~p"/")
+    assert html_response(conn, 200) =~ "Peace of mind from prototype to production"
+  end
+end