Better spawn error message (#1434)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Structure preview errors with source-aware messaging/UI and enhance
local/Docker spawn error diagnostics and logging.
> 
> - **Frontend**:
> - **Error model**: Change `previewErrorMessageAtom` from `string` to
`{ message, source }` to distinguish `preview-app` vs `dyad-app` errors.
> - **Preview UI**: Update `ErrorBanner` in
`components/preview_panel/PreviewIframe.tsx` to use `error.message`,
show an "Internal Dyad error" chip for `dyad-app`, conditional tip text,
and hide AI fix for non-`preview-app` errors; use `cn` helper.
> - **Error propagation**: Wrap iframe and build errors via
`setErrorMessage({ message, source: "preview-app" })`; adjust AI prompt
to use `errorMessage.message`.
> - **Hooks**:
> - `useRunApp`: On run/stop/restart failures, set `{ message, source:
"dyad-app" }` in `previewErrorMessageAtom`.
> - **Backend**:
> - `ipc/handlers/app_handlers.ts`: Improve spawn failure handling for
local node and Docker: capture stderr as strings, collect error details
(`message`, `code`, `errno`, `syscall`, `path`, `spawnargs`), log with
context, and throw enriched error messages.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
4135b04e19431dd53848c3266e5211e4c9df6aa2. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

src/atoms/appAtoms.ts

@@ -23,4 +23,6 @@ export const envVarsAtom = atom<Record<string, string | undefined>>({});
 
 export const previewPanelKeyAtom = atom<number>(0);
 
-export const previewErrorMessageAtom = atom<string | undefined>(undefined);
+export const previewErrorMessageAtom = atom<
+  { message: string; source: "preview-app" | "dyad-app" } | undefined
+>(undefined);