redirect /admin/theme to on-site editor at /?edit=theme

- Phase 5 was already implemented (URL mode activation via ?edit param)
- Phase 6: Add RedirectController to redirect /admin/theme → /?edit=theme
- Update admin sidebar and dashboard links to point directly to /?edit=theme
- Delete old Admin.Theme.Index LiveView and template (no longer needed)
- Update tests for new redirect behavior

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

lib/berrypod_web/router.ex

@@ -183,11 +183,8 @@ defmodule BerrypodWeb.Router do
       live "/redirects", Admin.Redirects, :index
     end
 
-    # Theme editor: admin root layout but full-screen (no sidebar)
-    live_session :admin_theme,
-      on_mount: [{BerrypodWeb.UserAuth, :require_authenticated}] do
-      live "/theme", Admin.Theme.Index, :index
-    end
+    # Theme editor redirects to on-site editing
+    get "/theme", RedirectController, :theme
   end
 
   # User account settings