Fix template rendering error in notes tab (Bug 001)

Add Errors field to HandleNotes data structure to match notes-tab
template expectations. The error-banner partial requires this field.

- Add template_test.go with reproduction test
- Update HandleNotes to include Errors field in data struct

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

2 files changed, 44 insertions, 2 deletions

diff --git a/internal/handlers/tabs.go b/internal/handlers/tabs.go
index 448dfbe..b8e7bcb 100644
--- a/internal/handlers/tabs.go
+++ b/internal/handlers/tabs.go
@@ -152,9 +152,11 @@ func (h *TabsHandler) HandleNotes(w http.ResponseWriter, r *http.Request) {
 	}
 
 	data := struct {
-		Notes []models.Note
+		Notes  []models.Note
+		Errors []string
 	}{
-		Notes: notes,
+		Notes:  notes,
+		Errors: nil,
 	}
 
 	// Check HX-Target header for partial update
diff --git a/internal/handlers/template_test.go b/internal/handlers/template_test.go
new file mode 100644
index 0000000..b0b2378
--- /dev/null
+++ b/internal/handlers/template_test.go
@@ -0,0 +1,40 @@
+package handlers_test
+
+import (
+	"html/template"
+	"io"
+	"testing"
+	"task-dashboard/internal/models"
+)
+
+func TestNotesTemplateRendering(t *testing.T) {
+	// Parse templates (adjust paths relative to where test runs, usually package root)
+	// Since we run 'go test ./...', paths might need to be absolute or relative to project root if we use a helper.
+	// But standard 'go test' in a subdir uses that subdir as CWD.
+	// We will assume the test runs from 'internal/handlers'.
+	// So paths are "../../web/templates/..."
+
+	tmpl, err := template.ParseGlob("../../web/templates/*.html")
+	if err != nil {
+		t.Fatalf("Failed to parse templates: %v", err)
+	}
+	tmpl, err = tmpl.ParseGlob("../../web/templates/partials/*.html")
+	if err != nil {
+		t.Fatalf("Failed to parse partials: %v", err)
+	}
+
+	// Define the data structure we EXPECT to use (with Errors)
+	data := struct {
+		Notes  []models.Note
+		Errors []string
+	}{
+		Notes:  []models.Note{},
+		Errors: []string{"Test Error"},
+	}
+
+	// Execute
+	err = tmpl.ExecuteTemplate(io.Discard, "notes-tab", data)
+	if err != nil {
+		t.Errorf("Failed to render notes-tab with corrected data: %v", err)
+	}
+}