Stop the sync poll from rebuilding the panel every 5 s

@Published fires even when the assigned value is unchanged, so the poll's
unconditional lastError = nil invalidated PanelView — which observes
SyncService — every 5 seconds, forever, hidden or visible. Over five days
that was ~185 CPU-minutes and ~190 MB, and clicks started to lag.

- Route poll state through setError, which skips equal values.
- Watch SyncService from a tiny SyncDot view instead of the panel body.
- Skip the URL cache on polls and give them timeouts.
- Only reassign the composer placeholder when it changed.

Sources/Prompt/ComposerView.swift

@@ -67,7 +67,9 @@ struct ComposerView: NSViewRepresentable {
             textView.string = text
             updateHeight(textView)
         }
-        textView.placeholderText = placeholder
+        if textView.placeholderText != placeholder {
+            textView.placeholderText = placeholder
+        }
         if isFocused, scrollView.window?.firstResponder != textView {
             scrollView.window?.makeFirstResponder(textView)
         }