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/site-settings/actions.ts

@@ -16,6 +16,7 @@ import {
   updateSiteSettings,
 } from "@/lib/app-config";
 import { getAdminAppPath, toInternalAdminPath } from "@/lib/admin-routing";
+import { withFlash } from "@/lib/admin-feedback";
 import {
   PAGE_TITLE_TOKEN,
   normalizeSiteDefaultLocale,
@@ -38,12 +39,6 @@ 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 parseJsonObject(rawValue: FormDataEntryValue | null, key: string) {
   if (typeof rawValue !== "string" || rawValue.trim() === "") {
@@ -260,7 +255,7 @@ export async function saveSiteBrandSettingsAction(formData: FormData) {
     });
 
     await revalidateSiteSettingsPages(parsedSettings.defaultLocale);
-    redirect(withMessage(getAdminAppPath("/site-settings/brand"), "success", "Einstellungen gespeichert."));
+    redirect(withFlash(getAdminAppPath("/site-settings/brand"), { success: "Einstellungen gespeichert." }));
   } catch (error) {
     if (isRedirectError(error)) {
       throw error;
@@ -273,7 +268,7 @@ export async function saveSiteBrandSettingsAction(formData: FormData) {
         ? error.message
         : "Einstellungen konnten nicht gespeichert werden.";
 
-    redirect(withMessage(getAdminAppPath("/site-settings/brand"), "error", message));
+    redirect(withFlash(getAdminAppPath("/site-settings/brand"), { error: message }));
   }
 }
 
@@ -323,7 +318,7 @@ export async function saveSiteLocalizationSettingsAction(formData: FormData) {
 
     await updateSiteSettings(parsedSettings);
     await revalidateSiteSettingsPages(parsedSettings.defaultLocale);
-    redirect(withMessage(getAdminAppPath("/site-settings/localization"), "success", "Einstellungen gespeichert."));
+    redirect(withFlash(getAdminAppPath("/site-settings/localization"), { success: "Einstellungen gespeichert." }));
   } catch (error) {
     if (isRedirectError(error)) {
       throw error;
@@ -334,6 +329,6 @@ export async function saveSiteLocalizationSettingsAction(formData: FormData) {
         ? error.message
         : "Einstellungen konnten nicht gespeichert werden.";
 
-    redirect(withMessage(getAdminAppPath("/site-settings/localization"), "error", message));
+    redirect(withFlash(getAdminAppPath("/site-settings/localization"), { error: message }));
   }
 }