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

src/client/utilities/DisableSafariPinchZoom.ts

@@ -13,16 +13,18 @@
  * (Chrome, Firefox, every Android browser) treat the listeners as a no-op,
  * so it is safe to install them unconditionally.
  *
+ * The listeners live for the document's lifetime; the browser releases them
+ * when the page is torn down, so no disposer is needed.
+ *
  * @param target - The EventTarget to attach the listeners to. Defaults to
  *   `document`, which is the scope Safari uses to decide whether to zoom
  *   the page.
- * @returns A function that removes the installed listeners.
  *
  * @see https://github.com/openfrontio/OpenFrontIO/issues/2330
  */
 export function installSafariPinchZoomBlocker(
   target: EventTarget = document,
-): () => void {
+): void {
   const block = (e: Event) => {
     e.preventDefault();
   };
@@ -31,10 +33,4 @@ export function installSafariPinchZoomBlocker(
   for (const type of events) {
     target.addEventListener(type, block);
   }
-
-  return () => {
-    for (const type of events) {
-      target.removeEventListener(type, block);
-    }
-  };
 }