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.

docs/frontend-system-current-state.md

@@ -215,11 +215,11 @@ Current feature code now reuses these levels in shared admin surfaces.
 
 Examples:
 
-- `components/root/media-field-picker.tsx`
-- `components/root/portfolio-projects-overview.tsx`
-- `components/root/portfolio-project-form.tsx`
-- `components/root/portfolio-categories-manager.tsx`
-- `components/root/site-settings-form.tsx`
+- `components/admin/media-field-picker.tsx`
+- `components/admin/portfolio-projects-overview.tsx`
+- `components/admin/portfolio-project-form.tsx`
+- `components/admin/portfolio-categories-manager.tsx`
+- `components/admin/site-settings-form.tsx`
 
 ## 6. Current input/button/select/dialog/sheet rules in use
 
@@ -485,7 +485,7 @@ Examples:
 - `components/ui/select.tsx`
 - `components/layout/site-header.tsx`
 - `components/theme-toggle.tsx`
-- `components/root/portfolio-project-form.tsx`
+- `components/admin/portfolio-project-form.tsx`
 
 No second React icon library is present in:
 
@@ -508,7 +508,7 @@ Remaining transitional or non-shared drift still present in the codebase:
   - `app/[locale]/(site)/portfolio/page.tsx`
   - `app/[locale]/(site)/portfolio/[slug]/page.tsx`
 - one custom dark preview surface remains in current admin code
-  - `components/root/site-settings-form.tsx`
+  - `components/admin/site-settings-form.tsx`
   - `bg-slate-950`
   - `text-white/55`
   - `text-white/60`