cleanup: drop unused disposer return from installSafariPinchZoomBlocker (#3992)

Follow-up to #3901 (cc @evanpelle).

## Description:

In the review on #3901, evanpelle pointed out that the disposer returned
by `installSafariPinchZoomBlocker` is never called at the call site in
`Main.ts`, and asked whether there's any reason to return it. There
isn't — the listeners live for the document's lifetime and the browser
releases them on teardown — so this PR drops the disposer.

### Changes

- `installSafariPinchZoomBlocker` now returns `void`. Removed the
`return () => { ... }` block and the `@returns` JSDoc line. Added a
sentence explaining why no disposer is needed.
- Tests: dropped the disposer-removal test, switched the behavior tests
to use fresh detached `<div>` elements (no document state leak across
tests), and verified the default-target = `document` case with
`vi.spyOn(document, 'addEventListener').mockImplementation(() => {})` so
no real listener actually attaches to the shared jsdom document.

Net diff: -23 lines (30 insertions, 53 deletions).

### What I tested

- `npm test` — 1245 + 65 tests pass, including the 4 surviving tests for
this helper
- `npm run build-prod` — succeeds (tsc + vite)
- `npx eslint` — clean
- `npx prettier --check` on the touched files — clean

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

@vansszh

tests/client/DisableSafariPinchZoom.test.ts

@@ -15,69 +15,50 @@ function dispatchCancelableGestureEvent(
 }
 
 describe("installSafariPinchZoomBlocker", () => {
-  it("prevents the default action of each Safari gesture event on document", () => {
-    const uninstall = installSafariPinchZoomBlocker();
-
-    try {
-      for (const type of GESTURE_EVENTS) {
-        const event = dispatchCancelableGestureEvent(document, type);
-        expect(event.defaultPrevented).toBe(true);
-      }
-    } finally {
-      uninstall();
-    }
-  });
-
-  it("attaches listeners to the provided target", () => {
+  it("calls preventDefault on each Safari gesture event dispatched at the target", () => {
     const target = document.createElement("div");
-    const uninstall = installSafariPinchZoomBlocker(target);
-
-    try {
-      for (const type of GESTURE_EVENTS) {
-        const event = dispatchCancelableGestureEvent(target, type);
-        expect(event.defaultPrevented).toBe(true);
-      }
-    } finally {
-      uninstall();
-    }
-  });
-
-  it("removes the listeners when the returned disposer is called", () => {
-    const target = document.createElement("div");
-    const uninstall = installSafariPinchZoomBlocker(target);
-    uninstall();
+    installSafariPinchZoomBlocker(target);
 
     for (const type of GESTURE_EVENTS) {
       const event = dispatchCancelableGestureEvent(target, type);
-      expect(event.defaultPrevented).toBe(false);
+      expect(event.defaultPrevented).toBe(true);
     }
   });
 
-  it("does not affect events on unrelated targets", () => {
-    const scope = document.createElement("div");
-    const other = document.createElement("div");
-    const uninstall = installSafariPinchZoomBlocker(scope);
+  it("defaults to attaching the listeners to document", () => {
+    const addEventListenerSpy = vi
+      .spyOn(document, "addEventListener")
+      .mockImplementation(() => {});
 
     try {
-      const event = dispatchCancelableGestureEvent(other, "gesturestart");
-      expect(event.defaultPrevented).toBe(false);
+      installSafariPinchZoomBlocker();
+
+      for (const type of GESTURE_EVENTS) {
+        expect(addEventListenerSpy).toHaveBeenCalledWith(
+          type,
+          expect.any(Function),
+        );
+      }
     } finally {
-      uninstall();
+      addEventListenerSpy.mockRestore();
     }
   });
 
+  it("does not affect events dispatched at unrelated targets", () => {
+    const scope = document.createElement("div");
+    const other = document.createElement("div");
+    installSafariPinchZoomBlocker(scope);
+
+    const event = dispatchCancelableGestureEvent(other, "gesturestart");
+    expect(event.defaultPrevented).toBe(false);
+  });
+
   it("leaves unrelated event types alone", () => {
-    const uninstall = installSafariPinchZoomBlocker();
+    const target = document.createElement("div");
+    installSafariPinchZoomBlocker(target);
 
-    try {
-      const event = new Event("touchstart", {
-        bubbles: true,
-        cancelable: true,
-      });
-      document.dispatchEvent(event);
-      expect(event.defaultPrevented).toBe(false);
-    } finally {
-      uninstall();
-    }
+    const event = new Event("touchstart", { bubbles: true, cancelable: true });
+    target.dispatchEvent(event);
+    expect(event.defaultPrevented).toBe(false);
   });
 });