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/mail/mail_test.go

@@ -0,0 +1,76 @@
+package mail
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestEmbeddedMarkIsPNG(t *testing.T) {
+	t.Parallel()
+
+	if !bytes.HasPrefix(markPNG, []byte("\x89PNG\r\n\x1a\n")) {
+		t.Fatal("embedded mark is not PNG data")
+	}
+}
+
+func TestQuestionAnsweredContent(t *testing.T) {
+	t.Parallel()
+
+	text, htmlBody := questionAnsweredContent("https://plumber.example/", QuestionAnswered{
+		ToName:        `<Sam & Pat>`,
+		QuestionID:    "question-123",
+		QuestionTitle: `<b>Leaky sink</b>`,
+		AnswerBody:    "Replace the cartridge.\nThen test the handle. <script>alert('x')</script>",
+	})
+
+	for _, want := range []string{
+		"Ask a Plumber First",
+		"Shop response",
+		"cid:answer-notification-mark",
+		"https://plumber.example/questions/question-123",
+		"white-space:pre-wrap",
+		"&lt;Sam &amp; Pat&gt;",
+		"&lt;b&gt;Leaky sink&lt;/b&gt;",
+		"&lt;script&gt;alert(&#39;x&#39;)&lt;/script&gt;",
+	} {
+		if !strings.Contains(htmlBody, want) {
+			t.Errorf("HTML missing %q", want)
+		}
+	}
+	for _, unsafe := range []string{
+		"<Sam & Pat>",
+		"<b>Leaky sink</b>",
+		"<script>alert('x')</script>",
+	} {
+		if strings.Contains(htmlBody, unsafe) {
+			t.Errorf("HTML contains unescaped content %q", unsafe)
+		}
+	}
+	if strings.Contains(htmlBody, "{{") {
+		t.Error("HTML contains an unresolved template token")
+	}
+	for _, want := range []string{
+		`Hi <Sam & Pat>,`,
+		`Your question "<b>Leaky sink</b>"`,
+		"Replace the cartridge.\nThen test the handle.",
+		"https://plumber.example/questions/question-123",
+	} {
+		if !strings.Contains(text, want) {
+			t.Errorf("text missing %q", want)
+		}
+	}
+}
+
+func TestQuestionAnsweredContentUsesFallbackTitle(t *testing.T) {
+	t.Parallel()
+
+	text, htmlBody := questionAnsweredContent("https://plumber.example", QuestionAnswered{})
+	if !strings.Contains(text, `"your question"`) {
+		t.Errorf("text missing fallback title")
+	}
+	if !strings.Contains(htmlBody, "a plumber replied to:</p>") ||
+		!strings.Contains(htmlBody, "“your question”") {
+		t.Errorf("HTML missing fallback title")
+	}
+}