Fix Discover Ticket panel add flow for tracked tickets

[Neokil]

Summary

• keep Discover Tickets open and add discovered tickets directly without bouncing through the manual add dialog
• filter out already tracked tickets when the panel opens and remove newly added tickets from the discover list immediately after acceptance
• cover the revised discover workflow with App and modal tests

Testing

• npm test
• npm run lint
• npm run typecheck
• make build
• go test ./...
• make test-e2e

Notes

• golangci-lint run in this worktree reports existing issues from sibling .auto-pr/worktrees/* paths outside GH-11, so it did not produce a clean repo-local signal for this change

Closes #11