test(17-01): add failing test + view model + message bubble CSS (RED)

- Append .message-row/.message-own/.message-other/.message-bubble/.message-meta CSS classes to app.css
- Create discussion_view.go with DiscussionMessageView, DiscussionTabData, NewDiscussionTabData
- Create discussion_view_test.go with TestChatMainContentRendersBubbleClasses (RED: compile error expected)

go-backend/internal/web/ui/app.css

@@ -1894,3 +1894,55 @@ td.text-right .borderless-icon-button.ui-icon-button-ghost.ui-icon-button-danger
     padding: 1.25rem;
   }
 }
+
+/* ── Message bubbles ── */
+
+.message-row {
+  padding: 0.75rem 1rem;
+}
+
+.message-own {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-end;
+}
+
+.message-other {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+}
+
+.message-bubble {
+  border: 1px solid var(--color-border-subtle);
+  border-radius: 0.25rem 0.75rem 0.75rem 0.75rem;
+  max-width: 70%;
+  padding: 0.75rem 1rem;
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
+.message-own .message-bubble {
+  background: rgba(128, 78, 236, 0.10);
+  border-radius: 0.75rem 0.75rem 0.25rem 0.75rem;
+}
+
+.message-other .message-bubble {
+  background: var(--color-surface-default);
+}
+
+.message-meta {
+  display: flex;
+  gap: 0.5rem;
+  margin-bottom: 0.25rem;
+  font-size: 0.875rem;
+  color: var(--color-text-muted);
+}
+
+.message-own .message-meta {
+  justify-content: flex-end;
+}
+
+.message-meta .message-author {
+  font-weight: 600;
+}

go-backend/internal/web/views/discussion_view.go

@@ -0,0 +1,47 @@
+package views
+
+// DiscussionMessageView holds the data for a single chat message row.
+type DiscussionMessageView struct {
+	Author    string
+	Timestamp string
+	Body      string
+	IsOwn     bool
+}
+
+// DiscussionTabData holds the full data set for the discussion tab.
+type DiscussionTabData struct {
+	Messages []DiscussionMessageView
+}
+
+// NewDiscussionTabData returns a DiscussionTabData populated with hardcoded demo
+// messages that alternate IsOwn values for visual verification of bubble styles.
+func NewDiscussionTabData() DiscussionTabData {
+	return DiscussionTabData{
+		Messages: []DiscussionMessageView{
+			{
+				Author:    "you@xtablo.com",
+				Timestamp: "09:14",
+				Body:      "Hey, any update on the design review?",
+				IsOwn:     true,
+			},
+			{
+				Author:    "other@xtablo.com",
+				Timestamp: "09:17",
+				Body:      "Just finished — sharing the file in the tablo now.",
+				IsOwn:     false,
+			},
+			{
+				Author:    "you@xtablo.com",
+				Timestamp: "09:19",
+				Body:      "Perfect, I'll take a look this afternoon.",
+				IsOwn:     true,
+			},
+			{
+				Author:    "other@xtablo.com",
+				Timestamp: "09:22",
+				Body:      "Sounds good. Let me know if you need changes.",
+				IsOwn:     false,
+			},
+		},
+	}
+}

go-backend/internal/web/views/discussion_view_test.go

@@ -0,0 +1,28 @@
+package views
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestChatMainContentRendersBubbleClasses(t *testing.T) {
+	data := DiscussionTabData{
+		Messages: []DiscussionMessageView{
+			{Author: "you@xtablo.com", Timestamp: "09:14", Body: "Hello", IsOwn: true},
+			{Author: "other@xtablo.com", Timestamp: "09:17", Body: "Hi there", IsOwn: false},
+		},
+	}
+
+	html := renderViewToString(t, ChatMainContent(data))
+
+	for _, want := range []string{
+		`class="ui-card"`,
+		`message-own`,
+		`message-other`,
+		`message-bubble`,
+	} {
+		if !strings.Contains(html, want) {
+			t.Fatalf("expected %q in rendered HTML, got:\n%s", want, html)
+		}
+	}
+}