fix(ui): show new run tabs immediately

[shenlvkang-collab]

Summary

• upsert and render newly created sessions before selecting them, without depending on session:created SSE timing
• fetch quick-start session details only when SSE has not already populated local state
• coalesce overlapping Run activations and keep the Run button disabled for at least 500 ms

Validation

• npm test -- test/run-mode-ui.test.ts (13 passed)
• npm run typecheck
• npm run lint
• npm run check:frontend-syntax
• npm run check:public-assets
• npm run format:check
• npm run build
• Playwright: double-clicking Run produced one immediate tab and one POST /api/sessions request

[Ark0N]

Thank you as always for the contribution! Nice design: keeping _onSessionCreated as the single idempotent upsert so POST-first and SSE-first orderings converge, plus the immediate tab render, is exactly the right shape, and the run() in-flight lock closes the double-click dupe. Verified locally: all 13 run-mode tests pass, typecheck/lint/frontend-syntax/public-assets/format all green, and I confirmed GET /api/sessions/:id goes through the preSerialization envelope so the fetch fallback parses correctly. One small note for a future pass: the welcome-screen buttons call runClaude()/runOpenCode()/runGemini() directly and so bypass the new lock; pre-existing behavior, not a blocker. Merging.