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/ARCHITECTURE.md

@@ -73,9 +73,6 @@ Handles media library queries and media bindings.
 lib/mail.ts  
 Handles SMTP delivery via nodemailer.
 
-lib/contact-guard.ts  
-Handles Turnstile verification and rate limiting for contact submissions.
-
 These modules act as the primary server-side application layer.
 
 ---
@@ -149,11 +146,7 @@ Database queries should not be implemented directly inside route-level UI files.
 app/[locale]/(site)/contact/page.tsx
 
 2. Server action validates the input using Zod.
-3. Turnstile verification and rate limiting run via:
-
-lib/contact-guard.ts
-
-4. Email is sent through:
+3. Email is sent through:
 
 lib/mail.ts