Studio Code: add recovery action to Session not found state

[sejas]

Related issues

• Fixes STU-2067

The Studio Code tab remembers one session id per site. When that session can no longer be resolved on disk (deleted, emptied, or corrupted transcript file), the tab dead-ended on a bare "Session not found" screen with no way out — restarting Studio or signing out didn't help because the stale reference persists locally.

How AI was used in this PR

AI helped trace the root cause, write the regression test, and draft this PR. All changes were reviewed manually.

Proposed Changes

This PR turns that dead end into a recovery path: the state now explains that the conversation is no longer available and offers a Start a new conversation button, which creates a fresh session for the site and replaces the stale reference. The screen also adopts the same layout and typography as the other empty states in this tab.

Testing Instructions

1. Open a site's Studio Code tab and confirm it loads normally.
2. Simulate a lost session: in DevTools run localStorage.setItem('studio_code_session_ids', JSON.stringify({ '<siteId>': 'bogus' })) (or delete the session's .jsonl file from the sessions directory), then reopen the tab, or break all your sessions by running this script in your chrome DevTools:

const key = 'studio_code_session_ids';
const map = JSON.parse(localStorage.getItem(key) ?? '{}');

for (const k of Object.keys(map)) {
  map[k] = crypto.randomUUID(); // replacement value per entry
}

localStorage.setItem(key, JSON.stringify(map));
console.log(JSON.parse(localStorage.getItem(key)));

4. Confirm the "Session not found" state appears with the explanation and the Start a new conversation button.
5. Click the button: a fresh, working conversation should load, and it should survive app restarts.
6. Check the state renders correctly in both light and dark mode.

Before	After

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing dd36ba0 vs trunk

app-size

Metric	trunk	dd36ba0	Diff	Change
App Size (Mac)	1353.02 MB	1353.02 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	dd36ba0	Diff	Change
load	1078 ms	1093 ms	+15 ms	⚪ 0.0%

site-startup

Metric	trunk	dd36ba0	Diff	Change
siteCreation	6993 ms	7017 ms	+24 ms	⚪ 0.0%
siteStartup	2356 ms	2351 ms	5 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[bcotrim]

LGTM 👍

Light	Dark