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/[locale]/(site)/contact/page.tsx

@@ -7,7 +7,7 @@ import { PageHero } from "@/components/layout/page-hero";
 import { MotionFade } from "@/components/motion-fade";
 import { ContactForm } from "@/components/site/contact-form";
 import { buildLocalizedMetadata } from "@/lib/metadata";
-import { getPublicContactProtectionSettings, getSiteSettings } from "@/lib/app-config";
+import { getSiteSettings } from "@/lib/app-config";
 import { getLocalizedPath, resolveLocale } from "@/lib/locale";
 import { AppCard } from "@/components/ui/app-card";
 import { CardContent, CardHeader, CardTitle } from "@/components/ui/card";
@@ -18,6 +18,9 @@ type ContactPageProps = {
   params: Promise<{
     locale: string;
   }>;
+  searchParams?: Promise<{
+    error?: string;
+  }>;
 };
 
 export async function generateMetadata({ params }: ContactPageProps): Promise<Metadata> {
@@ -34,14 +37,12 @@ export async function generateMetadata({ params }: ContactPageProps): Promise<Me
   });
 }
 
-export default async function ContactPage({ params }: ContactPageProps) {
+export default async function ContactPage({ params, searchParams }: ContactPageProps) {
   await params;
   const siteSettings = await getSiteSettings();
   const localeKey = resolveLocale(await getLocale().catch(() => siteSettings.defaultLocale), siteSettings.defaultLocale);
-  const [t, protection] = await Promise.all([
-    getTranslations({ locale: localeKey, namespace: "contactPage" }),
-    getPublicContactProtectionSettings(),
-  ]);
+  const t = await getTranslations({ locale: localeKey, namespace: "contactPage" });
+  const contactError = (await searchParams)?.error;
 
   return (
     <>
@@ -52,6 +53,17 @@ export default async function ContactPage({ params }: ContactPageProps) {
         description={t("intro")}
       />
 
+      {contactError ? (
+        <Container className="pb-6">
+          <p
+            role="alert"
+            className="rounded-nested border border-destructive/30 bg-destructive/10 px-4 py-3 text-sm font-medium text-destructive"
+          >
+            {contactError}
+          </p>
+        </Container>
+      ) : null}
+
       <Container className="grid gap-6 pb-12 lg:grid-cols-2 lg:pb-16">
         <MotionFade>
         <AppCard level={3}>
@@ -85,7 +97,6 @@ export default async function ContactPage({ params }: ContactPageProps) {
               action={submitContactFormAction}
               locale={localeKey}
               previewHref={getLocalizedPath(localeKey, "/success", siteSettings.defaultLocale)}
-              protection={protection}
               copy={{
                 name: t("name"),
                 email: t("email"),