feat: add Theme LiveView with preset switching

Implement basic theme editor interface with live preview:
- ThemeLive.Index LiveView with mount and event handlers
- Two-column layout: controls sidebar + preview area
- Display all 9 presets as clickable buttons
- Apply preset and regenerate CSS on click
- Show current theme settings (mood, typography, shape, density, color)
- Preview page switcher (7 pages: home, collection, product, cart, about, contact, 404)
- Inline <style> tag with generated CSS for instant preview
- Basic preview frame showing theme variables in action
- Authentication required via :require_authenticated_user pipeline
- Theme navigation link added to user menu
- 9 comprehensive LiveView tests

All tests passing (197 total).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

test/simpleshop_theme_web/live/theme_live_test.exs

@@ -0,0 +1,118 @@
+defmodule SimpleshopThemeWeb.ThemeLiveTest do
+  use SimpleshopThemeWeb.ConnCase, async: false
+
+  import Phoenix.LiveViewTest
+  import SimpleshopTheme.AccountsFixtures
+
+  alias SimpleshopTheme.Settings
+
+  setup do
+    user = user_fixture()
+    %{user: user}
+  end
+
+  describe "Index (unauthenticated)" do
+    test "redirects to login when not authenticated", %{conn: conn} do
+      {:error, redirect} = live(conn, ~p"/admin/theme")
+
+      assert {:redirect, %{to: path}} = redirect
+      assert path == ~p"/users/log-in"
+    end
+  end
+
+  describe "Index (authenticated)" do
+    setup %{conn: conn, user: user} do
+      conn = log_in_user(conn, user)
+      %{conn: conn}
+    end
+
+    test "renders theme editor page", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/admin/theme")
+
+      assert html =~ "Theme Editor"
+      assert html =~ "Save Theme"
+      assert html =~ "Presets"
+    end
+
+    test "displays all 9 presets", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/admin/theme")
+
+      assert html =~ "gallery"
+      assert html =~ "studio"
+      assert html =~ "boutique"
+      assert html =~ "bold"
+      assert html =~ "playful"
+      assert html =~ "minimal"
+      assert html =~ "night"
+      assert html =~ "classic"
+      assert html =~ "impulse"
+    end
+
+    test "displays current theme settings", %{conn: conn} do
+      {:ok, _settings} = Settings.apply_preset(:gallery)
+      {:ok, _view, html} = live(conn, ~p"/admin/theme")
+
+      assert html =~ "warm"
+      assert html =~ "editorial"
+      assert html =~ "soft"
+      assert html =~ "spacious"
+    end
+
+    test "displays generated CSS in preview", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/admin/theme")
+
+      assert html =~ ".preview-frame, .shop-root"
+      assert html =~ "--t-accent-h:"
+      assert html =~ "--t-surface-base:"
+      assert html =~ "--t-font-heading:"
+    end
+
+    test "applies preset and updates preview", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/theme")
+
+      html =
+        view
+        |> element("button", "gallery")
+        |> render_click()
+
+      theme_settings = Settings.get_theme_settings()
+      assert theme_settings.mood == "warm"
+      assert theme_settings.typography == "editorial"
+      assert html =~ "warm"
+      assert html =~ "editorial"
+    end
+
+    test "switches preview page", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/theme")
+
+      html =
+        view
+        |> element("button", "Collection")
+        |> render_click()
+
+      assert html =~ "Preview: Collection"
+    end
+
+    test "save theme button works", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/theme")
+
+      view
+      |> element("button", "Save Theme")
+      |> render_click()
+
+      assert view |> element("button", "Save Theme") |> has_element?()
+    end
+
+    test "all preview page buttons are present", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/admin/theme")
+
+      assert html =~ "Home"
+      assert html =~ "Collection"
+      assert html =~ "Product"
+      assert html =~ "Cart"
+      assert html =~ "About"
+      assert html =~ "Contact"
+      assert html =~ "404"
+    end
+  end
+end