test: guard fork-while-mid-turn in the web UI

[saucam]

Answers "I clicked fork while the model was still streaming and nothing happened."

Investigation (no bug found in the fork path)

• Daemon: smoke-tested session.fork while the parent is mid-turn — it returns response.ok, creates the fork with the history-so-far (forkedFrom.atTurn = user turns committed so far), the fork appears in session.list, and the parent turn keeps streaming and finishes cleanly (thinking → idle). Not gated on status.
• Web: ForkButton is always rendered/enabled (only its own in-flight busy() disables it); the working flag only arms the interrupt button. doFork sends session.fork and focuses the returned session regardless of turn status.

So fork-while-running works on both layers. The likely causes of "nothing happened" are unrelated to fork itself: (a) the earlier fork-to-openai-without-a-key case (already fixed by gating openai on its key), or (b) the multi-backend fork control is a dropdown — one click opens the menu, a second click on "fork (same backend)" / a target actually forks.

This PR

Locks the behavior in with a regression test: renders a focused session with status:"thinking" (asserts the interrupt button is enabled, proving it's genuinely mid-turn) and asserts the fork button is enabled and dispatches session.fork. Extends the sess() test helper with an optional status (defaults "idle").

Web suite: 211 pass, tsc clean.

Product note (not changed here)

Forking mid-turn branches at the last completed turn — the response currently streaming isn't in the fork yet (it's only committed to canonical history on turn_done). If we'd rather have mid-turn fork wait for the turn to finish (or snapshot the partial), that's a small follow-up — flag it if you want that behavior.

🤖 Generated with Claude Code

[github-actions]

🤖 Gemini code review

This PR adds a regression test to ensure that the session fork feature in the Web UI is not gated when the session is currently mid-turn (with a status of 'thinking'). It also enhances the sess test helper to allow specifying a session status, defaulting to 'idle'.

Findings: 🔴 0 · 🟠 0 · 🟡 0 · 🟢 0

---

_{Tokens spent · ⬆️ Input: 1,349 · ⬇️ Output: 73 · Σ Total: 2,133
Total may be higher due to thinking token counts.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.51%. Comparing base (ccf28d3) to head (ae9ed6a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #166   +/-   ##
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  Coverage   84.51%   84.51%           
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  Files         104      104           
  Lines       18319    18319           
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  Hits        15482    15482           
  Misses       2837     2837           

Flag	Coverage Δ
daemon	84.51% <ø> (ø)

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