Add issue creation form for repositories

[stylessh]

Summary

• Add a new issue creation page at /$owner/$repo/issues/new with a title input (gray bg, no shadow), full markdown editor with @mention support, and a sidebar with popover-based label and assignee pickers matching the PR detail sidebar pattern
• Add createIssue server function using octokit.rest.issues.create with support for title, body, labels, and assignees
• Add a "+" icon CTA to the Issues activity card header on the repo overview page linking to the new issue form
• On successful creation, invalidates relevant query caches and navigates to the newly created issue detail page

Test plan

• Navigate to a repo overview and verify the "+" icon appears in the Issues card header
• Click the "+" icon and verify it navigates to the new issue form
• Create an issue with title, body, labels, and assignees
• Verify the issue is created on GitHub and the user is redirected to the issue detail page
• Verify label and assignee popover pickers match the PR detail sidebar style

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	diffkit	79d7bea	Commit Preview URL Branch Preview URL	Apr 13 2026, 07:19 PM