test(17-02): add failing render test + view model + CSS h1 selector fix (RED)

- go-backend/internal/web/ui/app.css: prepend .overview-section-heading h1 to heading selector
- go-backend/internal/web/views/planning_view.go: PlanningEventRow, PlanningTabData, NewPlanningTabData (5 demo events / 2 dates), PlanningShowDaySeparator
- go-backend/internal/web/views/planning_view_test.go: TestPlanningShowDaySeparator (logic tests), TestPlanningMainContentRendersOverviewSection (render test — RED until Task 2)

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

@@ -883,6 +883,7 @@
   margin-bottom: 1.5rem;
 }
 
+.overview-section-heading h1,
 .overview-section-heading h3,
 .tasks-section-header h3 {
   color: var(--color-surface-muted-inverse);

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

@@ -0,0 +1,40 @@
+package views
+
+// PlanningEventRow holds data for a single event row in the planning agenda.
+type PlanningEventRow struct {
+	DateLabel  string
+	TimeRange  string
+	Title      string
+	TabloTitle string
+	Location   string
+}
+
+// PlanningTabData is the view model for the planning page.
+type PlanningTabData struct {
+	Events    []PlanningEventRow
+	DateRange string
+}
+
+// NewPlanningTabData returns a PlanningTabData with static demo events spanning 2 dates.
+func NewPlanningTabData() PlanningTabData {
+	return PlanningTabData{
+		DateRange: "May 17 – May 30, 2026",
+		Events: []PlanningEventRow{
+			{DateLabel: "May 17, 2026", TimeRange: "09:00-10:00", Title: "Sprint Planning", TabloTitle: "Product", Location: "Room A"},
+			{DateLabel: "May 17, 2026", TimeRange: "11:00-12:00", Title: "Design Review", TabloTitle: "Design", Location: ""},
+			{DateLabel: "May 17, 2026", TimeRange: "14:00-15:00", Title: "Stakeholder Sync", TabloTitle: "Product", Location: "HQ"},
+			{DateLabel: "May 18, 2026", TimeRange: "10:00-11:00", Title: "Engineering Stand-up", TabloTitle: "Engineering", Location: ""},
+			{DateLabel: "May 18, 2026", TimeRange: "15:00-16:00", Title: "Retrospective", TabloTitle: "Product", Location: "Room B"},
+		},
+	}
+}
+
+// PlanningShowDaySeparator returns true when a day separator header should be
+// rendered before the event at index. The separator is shown for the first
+// event (index 0) or whenever the date label changes from the previous event.
+func PlanningShowDaySeparator(events []PlanningEventRow, index int) bool {
+	if index == 0 {
+		return true
+	}
+	return events[index].DateLabel != events[index-1].DateLabel
+}

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

@@ -0,0 +1,54 @@
+package views
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+func TestPlanningShowDaySeparator(t *testing.T) {
+	events := []PlanningEventRow{
+		{DateLabel: "May 17, 2026", TimeRange: "09:00", Title: "A"},
+		{DateLabel: "May 17, 2026", TimeRange: "11:00", Title: "B"},
+		{DateLabel: "May 18, 2026", TimeRange: "10:00", Title: "C"},
+	}
+
+	tests := []struct {
+		name  string
+		index int
+		want  bool
+	}{
+		{"first event always shows separator", 0, true},
+		{"same date as previous — no separator", 1, false},
+		{"different date from previous — show separator", 2, true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := PlanningShowDaySeparator(events, tc.index)
+			if got != tc.want {
+				t.Fatalf("PlanningShowDaySeparator(events, %d) = %v; want %v", tc.index, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestPlanningMainContentRendersOverviewSection(t *testing.T) {
+	data := PlanningTabData{
+		DateRange: "May 17 – May 18, 2026",
+		Events: []PlanningEventRow{
+			{DateLabel: "May 17, 2026", TimeRange: "09:00", Title: "Alpha"},
+			{DateLabel: "May 17, 2026", TimeRange: "11:00", Title: "Beta"},
+			{DateLabel: "May 18, 2026", TimeRange: "10:00", Title: "Gamma"},
+		},
+	}
+
+	_ = context.Background() // ensure import is used
+	html := renderViewToString(t, PlanningMainContent(data))
+
+	for _, want := range []string{"overview-section", "overview-section-heading", "data-day-separator"} {
+		if !strings.Contains(html, want) {
+			t.Fatalf("PlanningMainContent rendered HTML missing %q; html: %.1500s", want, html)
+		}
+	}
+}