fix(flow-chat): preserve explicit session selection during restore

[limityan]

Summary

Fix session restore races where async historical initialization could overwrite a user-selected session or an active workspace after the user had already moved on.

Root cause

FlowChatManager.initializeWorkspace() auto-selected the latest historical session and awaited loadSessionHistory() before calling switchSession(). If the user clicked another session during that await, the stale initialization could still switch back to the auto-selected session. A stricter review also found the same class of issue at workspace scope: an older workspace initialization could continue after a newer workspace became active and then write old workspace/session side effects.

Changes

• Re-read the active session after historical restore completes and skip the stale auto-switch when the user selected another session during restore.
• Gate workspace initialization writes with a latest-request key so older workspace initialization cannot overwrite the newer active workspace path.
• Add AppLayout effect cleanup guards so stale initialization effects do not create sessions, send pending project descriptions, open pending settings, or show stale errors after workspace changes.
• Add unit coverage for same-workspace user selection and cross-workspace stale initialization.
• Add a user-only-latest E2E fixture scenario so historical sessions whose latest turn has no model response are validated against the latest visible content.

Validation

Check	Result
pnpm --dir src/web-ui run test:run src/flow_chat/services/FlowChatManager.test.ts	4 tests passed
pnpm run type-check:web	Passed
pnpm run check:repo-hygiene	Passed
pnpm run desktop:build:release-fast	Passed
release-fast controlled rapid switch E2E	4 tests passed; target latest visible 2772.0 ms, usable 2982.4 ms; blank/loading/scroll-jump/non-target hold counts all 0
release-fast user-only latest E2E	4 tests passed; first open latest visible 286.5 ms, usable 424.8 ms, full hydrate 1116.0 ms; blank/loading/scroll-jump/non-target hold counts all 0

Risk

• Normal initialization without user interaction still auto-selects the latest historical session.
• This does not change remote session loading, history hydration policy, startup gating, or rendering strategy.
• The E2E timing values above are validation observations for this correctness fix, not a claimed performance improvement.