refactor: drop toast + over-engineered extras, add inline admin feedback

Phase 1 cleanup of the personal-site revamp. Backend/architecture untouched;
changes are limited to removing unused complexity and restoring feedback.

Removals
- Toast system: delete react-hot-toast, Toaster, QueryToastBridge, lib/toast,
  the toggle/easter-egg calls, related i18n keys and the dependency.
- Contact protection: remove Turnstile + per-IP rate limiting
  (lib/contact-guard, lib/contact-protection, admin screen, form widget,
  app-config wiring, nav entry, test).
- Speculative specs: delete orders, products, downloads, project-inquiry.

Inline feedback (replaces toast, no new deps)
- Add lib/admin-feedback (withFlash/readFlash) and components/admin/admin-flash,
  rendered centrally by AdminDashboardShell.
- Emit success/error messages for media, site-settings, portfolio, smtp,
  marquee and maintenance actions; pages read them via searchParams.
- Contact form shows validation/delivery errors inline; success still
  redirects to /success.

Docs
- Fix stale paths in frontend-system-* (components/root -> components/admin,
  lib/root-navigation -> lib/admin-navigation, drop phantom src/) and remove
  contact-protection references from docs and CLAUDE.md.
- Add docs/PHASE0_DIAGNOSIS.md (diagnosis report).

Note: proxy.ts self-fetch kept intentionally; it also drives maintenance mode.

app/_admin/ui-kit/page.tsx

@@ -2,6 +2,7 @@ import { redirect } from "next/navigation";
 
 import { MotionFade } from "@/components/motion-fade";
 import { AdminDashboardShell } from "@/components/admin/admin-dashboard-shell";
+import { readFlash } from "@/lib/admin-feedback";
 import { clearAdminSessionCookie, isAdminAuthenticated } from "@/lib/admin-auth";
 import { getAdminAppPath } from "@/lib/admin-routing";
 import { UiKitShowcase } from "@/components/ui/ui-kit-showcase";
@@ -21,7 +22,13 @@ const copy = {
   backToSite: "Zur Website",
 };
 
-export default async function AdminUiKitPage() {
+export default async function AdminUiKitPage({
+  searchParams,
+}: {
+  searchParams?: Promise<{ success?: string; error?: string }>;
+}) {
+  const flash = readFlash(await searchParams);
+
   const authenticated = await isAdminAuthenticated();
 
   if (!authenticated) {
@@ -39,6 +46,7 @@ export default async function AdminUiKitPage() {
     <AdminDashboardShell
       copy={copy}
       active="ui-kit"
+      flash={flash}
       logoutAction={logoutAction}
       headerTitle={copy.title}
       headerDescription={copy.subtitle}