feat(v2): add admin GitHub App setup to Tools

[lilyshen0722]

Summary

• add an admin-only Install GitHub App action to the GitHub no-connection Tools row
• collect installationId, owner, and repo in the Tools aside and POST the existing admin route
• keep the affordance hidden for non-admins/connections, refresh after a new install, and explain when an existing installation belongs to another administrator
• defer all ten tools.* locale entries to feat(i18n): localize Connectors and Tools in zh-CN #1717 so that PR owns the single merged tools object; this branch uses English defaultValues until that native-pass push lands

The page check is cosmetic: a non-admin never sees the action, but the real authorization boundary is the existing POST /api/integrations/github-app route, protected by auth and adminAuth (after writeIntegrationsRateLimit). Repeated installs return the existing integration; the UI keeps the setup aside open and tells a different admin it is already owned.

Evidence

• before (non-admin, no setup action): docs/design/evidence/task-040-github-app-setup-before.png
• after at 1200px (admin action + setup aside): docs/design/evidence/task-040-github-app-setup-after-1200.png
• after at 390px (admin action + setup form): docs/design/evidence/task-040-github-app-setup-after-390.png

Verification

• npx jest src/v2/__tests__/V2ConnectorTools.test.tsx --runInBand (13 tests)
• npx jest src/v2/__tests__/v2-layout-invariants.test.ts --runInBand (108 tests)
• npx jest --runInBand (108 suites, 877 tests before deferring the ten locale keys to feat(i18n): localize Connectors and Tools in zh-CN #1717)
• npm run typecheck
• npm run build