Add polished answer notifications (#1)

Sends a branded Resend email when a question receives its first answer, records accepted and failed sends, and adds collapsed answer editing with cancel behavior.

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/store/email.go

@@ -0,0 +1,52 @@
+package store
+
+import (
+	"fmt"
+	"net/mail"
+	"strings"
+	"unicode/utf8"
+)
+
+const (
+	minEmailLen = 3
+	maxEmailLen = 254
+)
+
+// NormalizeEmail trims and lowercases an address for storage/comparison.
+func NormalizeEmail(s string) string {
+	return strings.ToLower(strings.TrimSpace(s))
+}
+
+// ValidateEmail returns a normalized address or an error message suitable for UI.
+func ValidateEmail(raw string) (normalized string, errMsg string) {
+	normalized = NormalizeEmail(raw)
+	if normalized == "" {
+		return "", "Email is required."
+	}
+	n := utf8.RuneCountInString(normalized)
+	if n < minEmailLen || len(normalized) > maxEmailLen {
+		return "", "Enter a valid email address."
+	}
+	addr, err := mail.ParseAddress(normalized)
+	if err != nil || addr.Address != normalized {
+		return "", "Enter a valid email address."
+	}
+	at := strings.LastIndex(normalized, "@")
+	if at < 1 || at == len(normalized)-1 {
+		return "", "Enter a valid email address."
+	}
+	domain := normalized[at+1:]
+	if !strings.Contains(domain, ".") || strings.HasPrefix(domain, ".") || strings.HasSuffix(domain, ".") {
+		return "", "Enter a valid email address."
+	}
+	return normalized, ""
+}
+
+// MustValidateEmail is like ValidateEmail but returns a Go error.
+func MustValidateEmail(raw string) (string, error) {
+	n, msg := ValidateEmail(raw)
+	if msg != "" {
+		return "", fmt.Errorf("%s", msg)
+	}
+	return n, nil
+}