✨ Add ability to delete issues

[Friedinger]

Problem

Currently, it is not possible to delete issues via the GitHub REST API (see discussion).
However, this functionality is available through the GitHub GraphQL API via the deleteIssue mutation.

We should implement a function in our codebase to allow deleting issues using GraphQL.

Proposed Implementation

• Use the graphql or graphql_flutter Dart package for sending GraphQL queries/mutations.
• Create a helper function to:
  1. Authenticate with a GitHub personal access token.
  2. Send the deleteIssue mutation with the required issueId.
  3. Handle success/failure responses appropriately.

Example mutation:

mutation {
  deleteIssue(input: {issueId: "ISSUE_ID"}) {
    clientMutationId
  }
}