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/live/admin/dashboard.ex

@@ -58,7 +58,7 @@ defmodule BerrypodWeb.Admin.Dashboard do
         <.link href={~p"/"} class="admin-btn admin-btn-primary">
           <.icon name="hero-arrow-top-right-on-square-mini" class="size-4" /> View your shop
         </.link>
-        <.link navigate={~p"/admin/theme"} class="admin-btn admin-btn-secondary">
+        <.link href="/?edit=theme" class="admin-btn admin-btn-secondary">
           <.icon name="hero-paint-brush-mini" class="size-4" /> Customise theme
         </.link>
       </div>
@@ -273,7 +273,7 @@ defmodule BerrypodWeb.Admin.Dashboard do
       %{
         key: :theme_customised,
         label: "Customise your theme",
-        href: "/admin/theme?from=checklist",
+        href: "/?edit=theme",
         hint: "Upload your logo, pick your colours, and choose a font that matches your brand."
       },
       %{