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.

AGENTS.md

@@ -177,6 +177,14 @@ brew upgrade --cask prompt
 
 ## Gotchas
 
+- **`@Published` publishes on every assignment, even when the value is
+  unchanged.** The 5-second sync poll used to assign `lastError = nil`
+  unconditionally; `PanelView` observes `SyncService`, so the entire panel
+  (rows, overlays, composer) rebuilt every 5 seconds, forever. Over five days
+  of uptime that was ~185 CPU-minutes and ~190 MB of footprint, and every
+  click started to lag. Poll state now goes through `setError`, which no-ops
+  when equal, and the header's sync dot is its own view so sync publishes
+  cannot invalidate the list.
 - **Never assign `NSApp.mainMenu`.** SwiftUI installs a menu whose Edit items
   are how AppKit routes Cmd+C/V/X/A/Z; replacing it with a hand-built menu
   silently breaks pasting, copying and select-all in the composer (this shipped