fix(settings): support multiple GitHub installations

[Bbbtt04]

Summary

• Render every GitHub App installation as an independently managed row with avatar, account type, and connector attribution.
• Keep Connect another GitHub available after the first installation and disconnect the exact selected row by UUID.
• Validate the installation list response with a forward-compatible schema and expose connected_by through the existing endpoint.

Root cause

The settings UI collapsed the installations array into a single connected state, hid the connect action, and always targeted installations[0]. The API also retained connected_by_id without exposing the connector name needed by the multi-row UI.

Permissions and behavior

• Owners and admins can connect and disconnect installations.
• Regular members see the complete read-only list without receiving numeric GitHub installation handles.
• Disconnect remains available when the workspace-level GitHub feature switch is off.
• Unknown future GitHub account types remain displayable instead of invalidating the list response.

Verification

• @multica/views: 234 test files, 2683 tests passed.
• @multica/core: 91 test files, 989 tests passed.
• Views and core TypeScript checks passed.
• Targeted ESLint checks passed.
• Full Go handler tests and go vet ./internal/handler passed.
• All four locale JSON files and git diff --check passed.
• Local Chrome flow passed with two seeded installations at desktop width and 394 px; the second row opened its own account-specific disconnect dialog and was cancelled without deleting data.

Closes #5599

[vercel]

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