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/marquee/actions.ts

@@ -6,6 +6,7 @@ import { isRedirectError } from "next/dist/client/components/redirect-error";
 
 import { getAdminAppPath, toInternalAdminPath } from "@/lib/admin-routing";
 import { clearAdminSessionCookie, isAdminAuthenticated } from "@/lib/admin-auth";
+import { withFlash } from "@/lib/admin-feedback";
 import { getSiteSettings, updateMarqueeSettings } from "@/lib/app-config";
 import { routing } from "@/i18n/routing";
 import { getLocalizedPath } from "@/lib/locale";
@@ -17,12 +18,6 @@ async function ensureAdmin() {
   }
 }
 
-function withMessage(pathname: string, type: "success" | "error", message: string) {
-  const params = new URLSearchParams();
-  params.set(type, message);
-
-  return `${pathname}?${params.toString()}`;
-}
 
 async function revalidateMarqueePages() {
   revalidatePath(toInternalAdminPath("/"));
@@ -73,13 +68,13 @@ export async function saveMarqueeSettingsAction(formData: FormData) {
     await updateMarqueeSettings(settings);
     await revalidateMarqueePages();
 
-    redirect(withMessage(getAdminAppPath("/marquee"), "success", "Marquee gespeichert."));
+    redirect(withFlash(getAdminAppPath("/marquee"), { success: "Marquee gespeichert." }));
   } catch (error) {
     if (isRedirectError(error)) {
       throw error;
     }
 
     const message = error instanceof Error ? error.message : "Marquee konnte nicht gespeichert werden.";
-    redirect(withMessage(getAdminAppPath("/marquee"), "error", message));
+    redirect(withFlash(getAdminAppPath("/marquee"), { error: message }));
   }
 }

app/_admin/marquee/page.tsx

@@ -2,6 +2,7 @@ import { redirect } from "next/navigation";
 
 import { MarqueeSettingsForm } from "@/components/admin/marquee-settings-form";
 import { AdminDashboardShell } from "@/components/admin/admin-dashboard-shell";
+import { readFlash } from "@/lib/admin-feedback";
 import { MotionFade } from "@/components/motion-fade";
 import { clearAdminSessionCookie, isAdminAuthenticated } from "@/lib/admin-auth";
 import { getAdminAppPath } from "@/lib/admin-routing";
@@ -26,7 +27,13 @@ const copy = {
   backToSite: "Zur Website",
 };
 
-export default async function AdminMarqueePage() {
+export default async function AdminMarqueePage({
+  searchParams,
+}: {
+  searchParams?: Promise<{ success?: string; error?: string }>;
+}) {
+  const flash = readFlash(await searchParams);
+
   if (!(await isAdminAuthenticated())) {
     redirect(getAdminAppPath("/"));
   }
@@ -44,6 +51,7 @@ export default async function AdminMarqueePage() {
     <AdminDashboardShell
       copy={copy}
       active="marquee"
+      flash={flash}
       logoutAction={logoutAction}
       headerTitle={copy.title}
       headerDescription={copy.subtitle}