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/page.tsx

@@ -21,6 +21,7 @@ import {
   setAdminSessionCookie,
 } from "@/lib/admin-auth";
 import { getAdminAppPath } from "@/lib/admin-routing";
+import { readFlash } from "@/lib/admin-feedback";
 import { getMaintenanceMode } from "@/lib/app-config";
 import { getAdminMediaAssets } from "@/lib/media";
 import { getAdminPortfolioCategories, getAdminPortfolioProjects } from "@/lib/portfolio";
@@ -28,6 +29,7 @@ import { getAdminPortfolioCategories, getAdminPortfolioProjects } from "@/lib/po
 type AdminPageProps = {
   searchParams?: Promise<{
     error?: string;
+    success?: string;
   }>;
 };
 
@@ -63,6 +65,7 @@ const copy = {
 
 export default async function AdminPage({ searchParams }: AdminPageProps) {
   const resolvedSearchParams = await searchParams;
+  const flash = readFlash(resolvedSearchParams);
   const authConfigured = isAdminAuthConfigured();
   const basicConfigured = Boolean(
     process.env.ADMIN_BASIC_AUTH_USER && process.env.ADMIN_BASIC_AUTH_PASS,
@@ -186,6 +189,7 @@ export default async function AdminPage({ searchParams }: AdminPageProps) {
     <AdminDashboardShell
       copy={copy}
       active="overview"
+      flash={flash}
       logoutAction={logoutAction}
       headerTitle={copy.title}
       headerDescription={copy.subtitle}