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/smtp/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 { isCheckedFormValue } from "@/lib/form-data";
 import {
   getMailSettings,
@@ -21,12 +22,6 @@ async function ensureAdmin() {
   }
 }
 
-function withMessage(pathname: string, type: "success" | "error", message: string) {
-  const params = new URLSearchParams();
-  params.set(type, message);
-
-  return `${pathname}?${params.toString()}`;
-}
 
 function parsePort(value: string) {
   const port = Number.parseInt(value, 10);
@@ -79,7 +74,7 @@ export async function saveMailSettingsAction(formData: FormData) {
 
     await updateMailSettings(nextMailSettings);
     revalidatePath(toInternalAdminPath("/smtp"));
-    redirect(withMessage(getAdminAppPath("/smtp"), "success", "SMTP Einstellungen gespeichert."));
+    redirect(withFlash(getAdminAppPath("/smtp"), { success: "SMTP Einstellungen gespeichert." }));
   } catch (error) {
     if (isRedirectError(error)) {
       throw error;
@@ -90,7 +85,7 @@ export async function saveMailSettingsAction(formData: FormData) {
         ? error.message
         : "SMTP Einstellungen konnten nicht gespeichert werden.";
 
-    redirect(withMessage(getAdminAppPath("/smtp"), "error", message));
+    redirect(withFlash(getAdminAppPath("/smtp"), { error: message }));
   }
 }
 
@@ -99,7 +94,7 @@ export async function sendTestEmailAction() {
 
   try {
     await sendTestEmail();
-    redirect(withMessage(getAdminAppPath("/smtp"), "success", "Test-E-Mail gesendet."));
+    redirect(withFlash(getAdminAppPath("/smtp"), { success: "Test-E-Mail gesendet." }));
   } catch (error) {
     if (isRedirectError(error)) {
       throw error;
@@ -110,6 +105,6 @@ export async function sendTestEmailAction() {
         ? error.message
         : "Test-E-Mail konnte nicht gesendet werden.";
 
-    redirect(withMessage(getAdminAppPath("/smtp"), "error", message));
+    redirect(withFlash(getAdminAppPath("/smtp"), { error: message }));
   }
 }