fix(app): devtools, session load churn, and stream batch flicker

- Add solid-devtools Vite plugin with locator (jsx/component locations) and
  @solid-devtools/overlay, loaded before render in dev only.
- Treat baseline message fetch as complete for empty transcripts so
  ensureSessionLoaded does not refetch on every prefetch/subagent ensure.
- When streaming delivers the first rows while non-idle, sync the batched
  message list immediately so the blueprint empty state does not flash.

Made-with: Cursor

apps/app/src/solid-devtools-dev.ts

@@ -0,0 +1,4 @@
+import "solid-devtools";
+import { attachDevtoolsOverlay } from "@solid-devtools/overlay";
+
+attachDevtoolsOverlay({ defaultOpen: false });