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.

CLAUDE.md

@@ -60,7 +60,7 @@ The full routing rewrite logic lives in `lib/admin-routing.ts` and `middleware.t
 
 Prisma client is in `lib/prisma.ts`. All DB access must go through server-side modules in `lib/`. Client components must never access Prisma.
 
-`AppConfig` is a key-value table used for all runtime configuration: site settings, SMTP, contact protection, marquee, maintenance mode, default locale. `lib/app-config.ts` is the aggregate entry point; individual settings are in `lib/site-settings.ts`, `lib/mail-settings.ts`, `lib/contact-protection.ts`, `lib/marquee-settings.ts`.
+`AppConfig` is a key-value table used for all runtime configuration: site settings, SMTP, marquee, maintenance mode, default locale. `lib/app-config.ts` is the aggregate entry point; individual settings are in `lib/site-settings.ts`, `lib/mail-settings.ts`, `lib/marquee-settings.ts`.
 
 ### Module boundaries
 
@@ -82,7 +82,7 @@ Prisma client is in `lib/prisma.ts`. All DB access must go through server-side m
 | AppConfig aggregate | `lib/app-config.ts` |
 | Portfolio queries | `lib/portfolio.ts` |
 | Media handling | `lib/media.ts` |
-| Contact flow | `lib/contact-guard.ts`, `lib/mail.ts` |
+| Contact flow | `lib/mail.ts` |
 
 ### Documentation to read by task scope