Closes #4384

Problem

After deleting a session from the history panel, the session sometimes reappears. CloseTab removes the tab from a.tabs and releases the lock before Snapshot() writes the session file. A concurrent DeleteSession checks a.tabs, doesn't see the tab, moves the file to trash — then Snapshot() recreates it at the original path.

Two resurrection vectors:

1. The explicit Snapshot() in CloseTab after delete(a.tabs, tabID)
2. The background tabSnapshotLoop goroutine that holds a raw pointer to the tab and calls Snapshot() independently

Why not just move Snapshot() inside the lock

PR #4400 takes this approach. It works for Vector 1 but has two problems:

• Snapshot() does disk I/O under a.mu, which freezes the UI during the write
• It does not fix Vector 2 — the autosave goroutine can still call Snapshot() after DeleteSession trashes the file, because it holds a captured session path from before SetSessionPath was cleared

What this fix does

Three changes, all in desktop/tabs.go:

1. Clear the controller's session path after the final snapshot (SetSessionPath). This makes all future Snapshot() calls no-ops. Verified safe: snapshot() at controller.go:2000 captures path under lock, returns early when path is empty.

2. Drain any in-flight autosave goroutine via a closing flag and sync.Cond (quiesceTabAutosave). CloseTab sets closing=true and waits until the loop finishes its current write. After quiesceTabAutosave returns, no background goroutine can call Snapshot again.

3. scheduleTabSnapshot checks the closing flag before launching new work, so no new autosave loops start after CloseTab begins teardown.

The detach path (hasActiveRuntimeWork and detachSessionRuntime) is exempt — detached runtimes keep running and must keep saving.

Disk I/O stays outside a.mu. The lock is not held during any file write.

Files Changed

• desktop/tabs.go — closing + saveCond fields on WorkspaceTab, quiesceTabAutosave(), scheduleTabSnapshot checks closing, tabSnapshotLoop broadcasts saveCond, CloseTab clears path + drains
• desktop/app_autosave_test.go — 2 regression tests

Verification

cd desktop and go build ./... and go vet ./...
cd desktop and go test -run TestCloseTab -v -count=1
cd desktop and go test -race -run TestCloseTab -v -count=1
cd desktop and go test -run TestTurnDone -v -count=1