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.

lib/admin-navigation.ts

@@ -26,7 +26,6 @@ type AdminNavigationCopy = {
   localizationSettings?: string;
   marquee?: string;
   smtp?: string;
-  contactProtection?: string;
 };
 
 export type AdminNavItem = {
@@ -41,7 +40,6 @@ export type AdminNavItem = {
 export function getAdminNavigation(
   copy: AdminNavigationCopy,
   active: "overview" | "maintenance" | "ui-kit" | "portfolio" | "media" | "site-settings" | "smtp" | "marquee",
-  smtpChild?: "settings" | "contact-protection",
   portfolioChild?: "overview" | "projects" | "new-project" | "categories",
   siteSettingsChild?: "brand" | "localization",
 ): AdminNavItem[] {
@@ -101,22 +99,7 @@ export function getAdminNavigation(
       label: copy.smtp ?? "SMTP",
       href: getAdminAppPath("/smtp"),
       icon: Mail,
-      active: active === "smtp" && !smtpChild,
-      expanded: active === "smtp",
-      children: [
-        {
-          label: copy.smtp ?? "SMTP",
-          href: getAdminAppPath("/smtp"),
-          icon: Mail,
-          active: smtpChild === "settings" || (!smtpChild && active === "smtp"),
-        },
-        {
-          label: copy.contactProtection ?? "Contact Protection",
-          href: getAdminAppPath("/smtp/contact-protection"),
-          icon: ShieldAlert,
-          active: smtpChild === "contact-protection",
-        },
-      ],
+      active: active === "smtp",
     },
     {
       label: copy.portfolio,