refactor(background): extract main world injection script

Move the 62-line inline main world injection function from background.ts
to a dedicated utils/rightClickInjection.ts file.

This separates the RightClickRestorer's DOM patching logic from the
service worker's message routing concerns.

- background.ts: -62 lines, now delegates to imported function
- utils/rightClickInjection.ts: new file with mainWorldInjectionScript()

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/utils/rightClickInjection.ts

@@ -0,0 +1,71 @@
+/**
+ * 右键解锁功能的主环境注入脚本
+ *
+ * 由于 content script 运行在 Isolated World，无法直接修改网页主环境的
+ * Event.prototype.preventDefault 等原生方法。需要通过 background
+ * 的 scripting.executeScript({ world: 'MAIN' }) 注入此脚本。
+ */
+
+/**
+ * 在网页主环境中执行的注入函数
+ * 注意：此函数会被序列化后通过 executeScript 注入，不能引用外部变量
+ */
+export function mainWorldInjectionScript(): void {
+  'use strict';
+  const w = window as unknown as Record<string, unknown>;
+  if (w.__testingToolsRightClickPatched) return;
+  w.__testingToolsRightClickPatched = true;
+
+  const PROTECTED = ['contextmenu', 'copy', 'paste', 'cut', 'selectstart'];
+
+  const _origPreventDefault = MouseEvent.prototype.preventDefault;
+  Object.defineProperty(MouseEvent.prototype, 'preventDefault', {
+    value: function (this: MouseEvent) {
+      const t = this.type;
+      if (PROTECTED.includes(t) || (t === 'mousedown' && this.button === 2)) {
+        return;
+      }
+      return _origPreventDefault.call(this);
+    },
+    writable: true,
+    configurable: true,
+  });
+
+  const _origStopPropagation = Event.prototype.stopPropagation;
+  Object.defineProperty(Event.prototype, 'stopPropagation', {
+    value: function (this: Event) {
+      if (PROTECTED.includes(this.type)) return;
+      return _origStopPropagation.call(this);
+    },
+    writable: true,
+    configurable: true,
+  });
+
+  const _origStopImmediatePropagation = Event.prototype.stopImmediatePropagation;
+  Object.defineProperty(Event.prototype, 'stopImmediatePropagation', {
+    value: function (this: Event) {
+      if (PROTECTED.includes(this.type)) return;
+      return _origStopImmediatePropagation.call(this);
+    },
+    writable: true,
+    configurable: true,
+  });
+
+  let _docOnContextMenu: unknown = null;
+  Object.defineProperty(document, 'oncontextmenu', {
+    get() {
+      return _docOnContextMenu;
+    },
+    set(fn: unknown) {
+      if (typeof fn === 'function') {
+        _docOnContextMenu = function (this: GlobalEventHandlers, e: MouseEvent) {
+          const r = (fn as (this: GlobalEventHandlers, ev: MouseEvent) => unknown).call(this, e);
+          return r === false ? true : r;
+        };
+      } else {
+        _docOnContextMenu = fn;
+      }
+    },
+    configurable: true,
+  });
+}