fix(desktop): keep a failed watch snapshot distinct from an empty directory

The storm-breaker poll mapped a throwing snapshot to '', which is also the
valid readdir snapshot of an empty directory, so an empty watched directory
vanishing after a trip was never reported. Failures are now null.

apps/desktop/electron/watch-storm-breaker.test.ts

@@ -179,4 +179,24 @@ describe('guardedWatch', () => {
 
     expect(onPollChange).toHaveBeenCalledTimes(1)
   })
+
+  test('an empty directory vanishing is still a change (error is not the empty snapshot)', () => {
+    let gone = false
+
+    const { fw, onPollChange } = setup('win32', {
+      snapshot: () => {
+        if (gone) {
+          throw new Error('ENOENT')
+        }
+
+        return ''
+      }
+    })
+
+    fw.emit(WATCH_STORM_MAX_EVENTS + 1)
+    gone = true
+    vi.advanceTimersByTime(WATCH_STORM_POLL_MS)
+
+    expect(onPollChange).toHaveBeenCalledTimes(1)
+  })
 })

apps/desktop/electron/watch-storm-breaker.ts

@@ -55,11 +55,16 @@ export function stormBreakerArmed(platform: string): boolean {
   return platform === 'win32'
 }
 
-function safeSnapshot(snapshot: () => string): string {
+/**
+ * `null` = the snapshot failed (target removed or unreadable). Kept distinct
+ * from the empty string, which is the valid snapshot of an empty directory, so
+ * an empty directory vanishing still compares as a change.
+ */
+function safeSnapshot(snapshot: () => string): string | null {
   try {
     return snapshot()
   } catch {
-    return ''
+    return null
   }
 }