Fix empty filename when Content-Disposition header is missing in /dl command

Add filenameFromURL helper to extract filename from URL path when
Content-Disposition header is empty or not present. This fixes the issue
where direct link downloads fail due to empty filename.

Co-authored-by: krau <71133316+krau@users.noreply.github.com>

core/tasks/directlinks/util_test.go

@@ -0,0 +1,78 @@
+package directlinks
+
+import (
+	"testing"
+)
+
+func TestFilenameFromURL(t *testing.T) {
+	tests := []struct {
+		name     string
+		url      string
+		expected string
+	}{
+		{
+			name:     "simple file",
+			url:      "https://example.com/file.zip",
+			expected: "file.zip",
+		},
+		{
+			name:     "file with path",
+			url:      "https://example.com/path/to/document.pdf",
+			expected: "document.pdf",
+		},
+		{
+			name:     "url with query params",
+			url:      "https://example.com/file.mp4?token=abc123",
+			expected: "file.mp4",
+		},
+		{
+			name:     "url with fragment",
+			url:      "https://example.com/file.txt#section",
+			expected: "file.txt",
+		},
+		{
+			name:     "url encoded filename",
+			url:      "https://example.com/%E6%B5%8B%E8%AF%95.zip",
+			expected: "测试.zip",
+		},
+		{
+			name:     "url encoded Chinese filename",
+			url:      "https://example.com/10%E6%9C%8817%E6%97%A5(6).mp4",
+			expected: "10月17日(6).mp4",
+		},
+		{
+			name:     "root path only",
+			url:      "https://example.com/",
+			expected: "",
+		},
+		{
+			name:     "no path",
+			url:      "https://example.com",
+			expected: "",
+		},
+		{
+			name:     "empty url",
+			url:      "",
+			expected: "",
+		},
+		{
+			name:     "file with spaces encoded",
+			url:      "https://example.com/my%20file.txt",
+			expected: "my file.txt",
+		},
+		{
+			name:     "complex path with multiple slashes",
+			url:      "https://cdn.example.com/a/b/c/d/e/video.mkv",
+			expected: "video.mkv",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := filenameFromURL(tt.url)
+			if result != tt.expected {
+				t.Errorf("filenameFromURL(%q) = %q, expected %q", tt.url, result, tt.expected)
+			}
+		})
+	}
+}