fix: extract title from h1 or URL slug when page title starts with #

When readallcomics.com pages have a <title> containing only the issue
number (e.g. '#018 (2026)'), fall back to the h1 element first, then
derive the title from the URL slug by stripping the trailing year and
title-casing the hyphen-separated segments.

Closes #4

comic/download_test.go

@@ -0,0 +1,145 @@
+package comic
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestComicDownloadError(t *testing.T) {
+	err := ComicDownloadError{Message: "download failed", Code: 1}
+	if err.Error() != "download failed" {
+		t.Errorf("Error() = %q, want %q", err.Error(), "download failed")
+	}
+}
+
+func TestHandleRequest(t *testing.T) {
+	t.Run("successful request", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Header.Get("User-Agent") == "" {
+				t.Error("expected User-Agent header to be set")
+			}
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte("image data"))
+		}))
+		defer server.Close()
+
+		resp, err := handleRequest(server.URL)
+		if err != nil {
+			t.Fatalf("handleRequest() unexpected error: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("handleRequest() status = %d, want %d", resp.StatusCode, http.StatusOK)
+		}
+	})
+
+	t.Run("non-200 response", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusNotFound)
+		}))
+		defer server.Close()
+
+		_, err := handleRequest(server.URL)
+		if err == nil {
+			t.Error("handleRequest() expected error for 404 response, got nil")
+		}
+	})
+
+	t.Run("invalid URL", func(t *testing.T) {
+		_, err := handleRequest("http://invalid.localhost:0/bad")
+		if err == nil {
+			t.Error("handleRequest() expected error for invalid URL, got nil")
+		}
+	})
+}
+
+func TestDownloadFile(t *testing.T) {
+	t.Run("successful download", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte("fake image content"))
+		}))
+		defer server.Close()
+
+		tmpDir := t.TempDir()
+		c := &Comic{
+			Title:       "TestComic",
+			LibraryPath: tmpDir,
+		}
+
+		err := downloadFile(server.URL+"/image.jpg", 1, c)
+		if err != nil {
+			t.Fatalf("downloadFile() unexpected error: %v", err)
+		}
+
+		expectedPath := filepath.Join(tmpDir, "TestComic", "TestComic 001.jpg")
+		if _, err := os.Stat(expectedPath); os.IsNotExist(err) {
+			t.Errorf("expected file %s to exist", expectedPath)
+		}
+	})
+
+	t.Run("formats page number with leading zeros", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte("fake image content"))
+		}))
+		defer server.Close()
+
+		tmpDir := t.TempDir()
+		c := &Comic{
+			Title:       "TestComic",
+			LibraryPath: tmpDir,
+		}
+
+		err := downloadFile(server.URL+"/image.jpg", 42, c)
+		if err != nil {
+			t.Fatalf("downloadFile() unexpected error: %v", err)
+		}
+
+		expectedPath := filepath.Join(tmpDir, "TestComic", "TestComic 042.jpg")
+		if _, err := os.Stat(expectedPath); os.IsNotExist(err) {
+			t.Errorf("expected file %s to exist", expectedPath)
+		}
+	})
+
+	t.Run("server error returns error", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusInternalServerError)
+		}))
+		defer server.Close()
+
+		tmpDir := t.TempDir()
+		c := &Comic{
+			Title:       "TestComic",
+			LibraryPath: tmpDir,
+		}
+
+		err := downloadFile(server.URL+"/image.jpg", 1, c)
+		if err == nil {
+			t.Error("downloadFile() expected error for server error, got nil")
+		}
+	})
+
+	t.Run("empty response body returns error", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			// write nothing
+		}))
+		defer server.Close()
+
+		tmpDir := t.TempDir()
+		c := &Comic{
+			Title:       "TestComic",
+			LibraryPath: tmpDir,
+		}
+
+		err := downloadFile(server.URL+"/image.jpg", 1, c)
+		if err == nil {
+			t.Error("downloadFile() expected error for empty body, got nil")
+		}
+	})
+}