Error Feedback Service + friendly error handling across all pages

- Error Feedback Service: POST /api/feedback saves to JSONL, GET with admin token
- ErrorFeedback component: warm amber tones, friendly messages, 'Report Issue' button
- Replaced ALL red-themed error displays across 15+ pages with ErrorFeedback
- Kind classification: network, auth, location, upgrade, default messages
- QA test suite v2: 8-layer testing (system, API, render, scenario, edge, mobile, perf, security)
- Browser-based visual QA with Playwright (Layer 9)

src/app/auth/page.tsx

@@ -4,6 +4,7 @@ import { Suspense, useState, FormEvent, useEffect } from "react";
 import { useAuth } from "@/lib/AuthContext";
 import { useRouter, useSearchParams } from "next/navigation";
 import { Mail, ArrowLeft } from "lucide-react";
+import ErrorFeedback from "@/components/ErrorFeedback";
 
 type AuthMode = "login" | "register";
 type PageState = "select" | "email-form";
@@ -222,9 +223,7 @@ function AuthPageInner() {
 
         {/* Error */}
         {error && (
-          <div className="mb-4 p-3 rounded-xl bg-red-900/20 border border-red-800/40 text-red-400 text-sm text-center">
-            {error}
-          </div>
+          <ErrorFeedback error={error} kind="auth" onRetry={() => setError("")} context="authentication" />
         )}
 
         {/* Form */}