add HTMLImageElement.complete property

Per spec, complete returns true when the image has no src, is fully
fetched, or is broken. Since this is a headless browser that does
not fetch images, complete always returns true.

src/browser/tests/element/html/image.html

@@ -98,6 +98,22 @@
 }
 </script>
 
+<script id="complete">
+{
+    // Image with no src is complete per spec
+    const img = document.createElement('img');
+    testing.expectEqual(true, img.complete);
+
+    // Image with src is also complete (headless browser, no actual fetch)
+    img.src = 'test.png';
+    testing.expectEqual(true, img.complete);
+
+    // Image constructor also complete
+    const img2 = new Image();
+    testing.expectEqual(true, img2.complete);
+}
+</script>
+
 <script id="load-trigger-event">
 {
   const img = document.createElement("img");

src/browser/webapi/element/html/Image.zig

@@ -112,6 +112,12 @@ pub fn getNaturalHeight(_: *const Image) u32 {
     return 0;
 }
 
+pub fn getComplete(_: *const Image) bool {
+    // Per spec, complete is true when: no src/srcset, image fully fetched,
+    // or image is broken. Since we don't fetch images, they are always complete.
+    return true;
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(Image);
 
@@ -131,6 +137,7 @@ pub const JsApi = struct {
     pub const loading = bridge.accessor(Image.getLoading, Image.setLoading, .{});
     pub const naturalWidth = bridge.accessor(Image.getNaturalWidth, null, .{});
     pub const naturalHeight = bridge.accessor(Image.getNaturalHeight, null, .{});
+    pub const complete = bridge.accessor(Image.getComplete, null, .{});
 };
 
 pub const Build = struct {