fall back to createFlow when the update target does not exist (#10206)

FlowBuilder's deploy branches solely on the `newFlow` prop. When an embedder
mounts the builder with `newFlow` false for a flow that was never deployed,
Deploy issues `PUT /flows/update/{initialPath}` and the backend answers
`Flow not found at name <path>`, so the first deploy can never land.

Confirm against `GET /flows/exists/{path}` before taking the update branch,
and create instead when nothing is deployed there. The create path and the
full-page editor (where `newFlow` is derived from `no_deployed`) are
unchanged.

Fixes WIN-2206

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/FlowBuilder.svelte

@@ -413,7 +413,22 @@
 			// loadingSave = false // del
 			// return
 
-			if (newFlow) {
+			// `newFlow` comes from the embedder, and updating a path that has no
+			// deployed flow 404s. Confirm with the server before taking the update
+			// branch so a first deploy still lands.
+			let isNewFlow = newFlow
+			if (!isNewFlow) {
+				try {
+					isNewFlow =
+						initialPath === '' ||
+						!(await FlowService.existsFlowByPath({ workspace: opWorkspace!, path: initialPath }))
+				} catch (err) {
+					// Unreachable check: keep the caller's intent rather than failing the deploy.
+					console.error('Could not check flow existence', err)
+				}
+			}
+
+			if (isNewFlow) {
 				await FlowService.createFlow({
 					workspace: opWorkspace!,
 					requestBody: {