GHSA-269c-h76q-8cxw

### Summary

A page editor without `admin.super` can place an event handler after a `>` inside a quoted attribute. Grav accepts and stores the page, then executes the handler in the application origin when a visitor opens it.

### Details

`Security::detectXss()` (`system/src/Grav/Common/Security.php:253`) anchors the `on_events` scan at `<` and uses `[^>]*?`, which cannot cross the first literal `>`. When that character is inside a quoted value, the browser keeps the tag open and parses the later `onerror` attribute, so the detector and browser disagree. `AdminController::savePage()` relies on this detector when saving content from page editors outside the `admin.super` whitelist.

### PoC

I reproduced this with `getgrav/grav` 2.0.11 (`ad9709f865b09b68798fb1ac375b484a8cc1d892`), Admin 1.10.52, and Quark 2 1.1.4.

1. Sign in as a user with `admin.login` and `admin.pages`, but without `admin.super`. 2. Create or edit `/xsstest` and save this page body:

```html <img src=x title=">" onerror=alert(document.domain)> ```

3. Open `/xsstest` in a private browser window.

The save succeeds and the visitor sees an alert containing the site domain. With the body changed to `<img src=x onerror=alert(1)>`, the same endpoint rejects it with `XSS issue detected` and does not store it.

### Impact

A page editor can execute JavaScript in the origin of every user who views the stored page, including unauthenticated visitors.

### Anticipated objection and response

Although the `detectXss()` docblock describes it as a heuristic that cannot catch every XSS, this check is the storage-time boundary for page editors outside the default `security.xss_whitelist` of `admin.super`. The same endpoint rejects a plain handler but accepts this executable form, allowing a lower-trust editor to cross the boundary the check is intended to enforce.

### Suggested fix

Prefer an HTML tokenizer or sanitizer that rejects event-handler attributes on parsed elements. If the existing tripwire remains, make its tag scan quote-aware instead of treating every `>` as a boundary. Add double-quoted and single-quoted regression cases plus the rejected plain-handler control.