fix: Move HandlePlatformFileDrop to window._wails internal API (#4616)

* fix: Move HandlePlatformFileDrop to window._wails internal API

Fixes #4489

## Problem
The npm package @wailsio/runtime doesn't work for drag-and-drop because
Go backend calls window.wails.Window.HandlePlatformFileDrop(), but the
npm package doesn't populate window.wails (by design for encapsulation).

## Root Cause
PR #3295 (March 2024) intentionally removed window.wails assignment from
the npm package to improve encapsulation. However, this broke platform
handlers that Go backend relies on.

## Solution
Move HandlePlatformFileDrop from public API (window.wails) to internal
API (window._wails), following the existing pattern:
- window._wails.invoke
- window._wails.environment
- window._wails.flags

## Changes
- Register handlePlatformFileDrop in window._wails namespace
- Update Go backend to call window._wails.handlePlatformFileDrop()
- Use camelCase naming for consistency with other internal API methods
- Rebuild bundled runtime with changes

## Benefits
✅ npm package works without window.wails pollution
✅ Maintains encapsulation of public API
✅ Platform handlers clearly separated as internal
✅ Follows existing internal API conventions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* chore: Add changelog entry for drag-and-drop fix

Updates UNRELEASED_CHANGELOG.md with the fix for #4489.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

v3/UNRELEASED_CHANGELOG.md

@@ -23,7 +23,7 @@ After processing, the content will be moved to the main changelog and this file
 
 ## Fixed
 <!-- Bug fixes -->
-- Fix the white window appearing on Windows when creating a hidden window by @leaanthony in [#4612](https://github.com/wailsapp/wails/pull/4612)
+- Fix drag-and-drop not working when using npm package @wailsio/runtime (#4489) by @leaanthony in #4616
 
 ## Deprecated
 <!-- Soon-to-be removed features -->

v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts

@@ -54,4 +54,10 @@ export * from "./cancellable.js";
 
 // Notify backend
 window._wails.invoke = System.invoke;
+
+// Register platform handlers (internal API)
+// Note: Window is the thisWindow instance (default export from window.ts)
+// Binding ensures 'this' correctly refers to the current window instance
+window._wails.handlePlatformFileDrop = Window.HandlePlatformFileDrop.bind(Window);
+
 System.invoke("wails:runtime:ready");

v3/pkg/application/webview_window.go

@@ -1498,7 +1498,7 @@ func (w *WebviewWindow) InitiateFrontendDropProcessing(filenames []string, x int
 	}
 
 	jsCall := fmt.Sprintf(
-		"window.wails.Window.HandlePlatformFileDrop(%s, %d, %d);",
+		"window._wails.handlePlatformFileDrop(%s, %d, %d);",
 		string(filenamesJSON),
 		x,
 		y,