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/components/layouts/admin.html.heex

@@ -143,10 +143,7 @@
               </.link>
             </li>
             <li>
-              <.link
-                href={~p"/admin/theme"}
-                class={admin_nav_active?(@current_path, "/admin/theme")}
-              >
+              <.link href="/?edit=theme">
                 <.icon name="hero-paint-brush" class="size-5" /> Theme
               </.link>
             </li>