perf: self-host fonts and add /admin route

Self-hosted fonts:
- Download all 10 typefaces (35 font files, 728KB) from Google Fonts
- Create @font-face declarations in assets/css/fonts.css
- Remove Google Fonts external dependency from layouts
- Privacy improvement (no Google tracking)
- Performance improvement (no DNS lookup to fonts.googleapis.com)
- GDPR compliant (no third-party requests)

Admin access:
- Add /admin route that redirects to /admin/theme (requires auth)
- Remove Admin link from footer (too visible for visitors)
- Shop owners can bookmark or type /admin directly

Layout improvements:
- Create shop_root.html.heex as minimal root for shop pages
- Shop pages no longer show admin nav bar

Other:
- Update .gitignore to exclude digested static files
- Add PageSpeed 100% task to ROADMAP.md
- Fix test to check /users/settings instead of shop homepage

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

test/simpleshop_theme_web/controllers/user_session_controller_test.exs

@@ -20,8 +20,8 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert get_session(conn, :user_token)
       assert redirected_to(conn) == ~p"/"
 
-      # Now do a logged in request and assert on the menu
-      conn = get(conn, ~p"/")
+      # Now do a logged in request to an admin page and assert on the menu
+      conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
       assert response =~ ~p"/users/settings"
@@ -84,8 +84,8 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert get_session(conn, :user_token)
       assert redirected_to(conn) == ~p"/"
 
-      # Now do a logged in request and assert on the menu
-      conn = get(conn, ~p"/")
+      # Now do a logged in request to an admin page and assert on the menu
+      conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
       assert response =~ ~p"/users/settings"
@@ -108,8 +108,8 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
 
       assert Accounts.get_user!(user.id).confirmed_at
 
-      # Now do a logged in request and assert on the menu
-      conn = get(conn, ~p"/")
+      # Now do a logged in request to an admin page and assert on the menu
+      conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
       assert response =~ ~p"/users/settings"