Fix workspace navigation regression on web (#772)

Centralize workspace navigation behind a small route-derived API so the workspace shell reads active selection from the current Expo Router route instead of a separately synchronized store.

Remove the stale layout-driven active-workspace write that regressed web after 02e5b564.

packages/app/src/screens/new-workspace-screen.tsx

@@ -377,7 +377,6 @@ function submitWorkspaceDraft(input: SubmitDraftInput): void {
     serverId,
     workspaceId,
     target: { kind: "draft", draftId },
-    navigationMethod: "replace",
   });
   useDraftStore.getState().clearDraftInput({ draftKey, lifecycle: "sent" });
 }