fix(v3): implement file picker for input type=file on macOS (#4862)

Add WKUIDelegate protocol support to handle runOpenPanelWithParameters
delegate method, which is required for <input type="file"> elements to
work in WKWebView on macOS. This was a regression from V2 which had
this implementation.

Changes:
- Add WKUIDelegate to WebviewWindowDelegate protocol list
- Set UIDelegate on WKWebView during window creation
- Implement runOpenPanelWithParameters to show NSOpenPanel for file selection

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

v3/UNRELEASED_CHANGELOG.md

@@ -32,6 +32,7 @@ After processing, the content will be moved to the main changelog and this file
 - Make menus to be displayed on Windows OS in `v3\examples\dialogs` by @ndianabasi
 - Fix race condition causing TypeError during page reload (#4872) by @ddmoney420
 - Fix incorrect output from binding generator tests by removing global state in the `Collector.IsVoidAlias()` method (#4941) by @fbbdev
+- Fix `<input type="file">` file picker not working on macOS (#4862) by @leaanthony
 
 ## Deprecated
 <!-- Soon-to-be removed features -->

v3/pkg/application/webview_window_darwin.go

@@ -111,6 +111,7 @@ void* windowNew(unsigned int id, int width, int height, bool fraudulentWebsiteWa
 
     // support webview events
     [webView setNavigationDelegate:delegate];
+    [webView setUIDelegate:delegate];
 
 	// Ensure webview resizes with the window
 	[webView setAutoresizingMask:NSViewWidthSizable | NSViewHeightSizable];

v3/pkg/application/webview_window_darwin.h

@@ -18,7 +18,7 @@
 
 @end
 
-@interface WebviewWindowDelegate : NSObject <NSWindowDelegate, WKScriptMessageHandler, WKNavigationDelegate, WKURLSchemeHandler, NSDraggingDestination>
+@interface WebviewWindowDelegate : NSObject <NSWindowDelegate, WKScriptMessageHandler, WKNavigationDelegate, WKURLSchemeHandler, NSDraggingDestination, WKUIDelegate>
 
 @property unsigned int windowId;
 @property (retain) NSEvent* leftMouseEvent;

v3/pkg/application/webview_window_darwin.m

@@ -780,6 +780,23 @@ typedef NS_ENUM(NSInteger, MacLiquidGlassStyle) {
     }
 }
 // GENERATED EVENTS END
+
+// WKUIDelegate - Handle file input element clicks
+- (void)webView:(WKWebView *)webView runOpenPanelWithParameters:(WKOpenPanelParameters *)parameters
+    initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(NSArray<NSURL *> * URLs))completionHandler {
+    NSOpenPanel *openPanel = [NSOpenPanel openPanel];
+    openPanel.allowsMultipleSelection = parameters.allowsMultipleSelection;
+    if (@available(macOS 10.14, *)) {
+        openPanel.canChooseDirectories = parameters.allowsDirectories;
+    }
+    [openPanel beginSheetModalForWindow:webView.window
+        completionHandler:^(NSInteger result) {
+            if (result == NSModalResponseOK)
+                completionHandler(openPanel.URLs);
+            else
+                completionHandler(nil);
+        }];
+}
 @end
 void windowSetScreen(void* window, void* screen, int yOffset) {
     WebviewWindow* nsWindow = (WebviewWindow*)window;