ErrorPage crash reports are filed against upstream Headlamp, not AKS Desktop

## Problem

`ErrorPage`'s "Open Issue on GitHub" button files crash reports against
**upstream Headlamp**, not AKS Desktop:

```ts
// headlamp/frontend/src/components/common/ErrorPage/ErrorPage.tsx
const GITHUB_REPO_URL = 'https://github.com/kubernetes-sigs/headlamp';
```

The page around it is already AKS-branded (`t('Your AKS desktop ran into a
problem!')`), so the destination is the odd one out. AKS Desktop crash reports
land in a repo whose maintainers can't act on them.

Raised by @yolossn and @illume on #843.

## Proposed fix

Point it at this repo and align the label with the new "Contact us" vocabulary:

```ts
const GITHUB_REPO_URL = 'https://github.com/Azure/aks-desktop';
```

**Keep the structured `issues/new?title=…&body=…` URL.** The reviewers on #843
asked for the same URL as the "Contact us" button, but that button's destination
(`aka.ms/aks-desktop/feedback` → a Microsoft Forms page) cannot receive the
prefilled crash report — error message, truncated stack, app version, git commit,
user agent, platform. Unifying the URL literally would discard all of it and leave
users pasting stack traces by hand.

So: unify the **text**, fix the **repo**, keep the **structured URL**.

## Alternative

If one destination matters more than diagnostic fidelity, point the button at
`CONTACT_US_URL` and drop `handleOpenGitHubIssue` entirely. Worth an explicit
decision rather than a default.

## Notes

- This is a `headlamp/` submodule change — needs a PR against `headlamp-downstream`
plus a submodule pointer bump. Deliberately kept out of #843 to avoid turning a
plugin PR into a cross-repo one.
- There is no plugin API to override `ErrorPage`'s crash button, so the fork edit is
justified. Use the `aksd:` prefix (AKS-specific branding, not upstreamable).
- Changing the label will require regenerating
`frontend/src/components/common/ErrorPage/__snapshots__/ErrorPage.WithErrorStack.stories.storyshot`.

Start in headlamp/frontend/src/components/common/ErrorPage/ErrorPage.tsx and inspect the existing crash-report URL and label, then review the Contact us destination and the structured issues/new parameters. Update the fork through the headlamp submodule, regenerate frontend/src/components/common/ErrorPage/__snapshots__/ErrorPage.WithErrorStack.stories.storyshot, and bump the submodule pointer in headlamp-downstream; done means AKS Desktop reports retain their diagnostic fields while using the AKS Desktop repository and label.

Written by the indexing model from the issue text.