feat: persist diff panel state across thread navigation (#875)

Author: eggfriedrice24

apps/web/src/routes/_chat.$threadId.tsx

@@ -1,10 +1,14 @@
 import { ThreadId } from "@t3tools/contracts";
-import { createFileRoute, useNavigate } from "@tanstack/react-router";
+import { createFileRoute, retainSearchParams, useNavigate } from "@tanstack/react-router";
 import { Suspense, lazy, type ReactNode, useCallback, useEffect } from "react";
 
 import ChatView from "../components/ChatView";
 import { useComposerDraftStore } from "../composerDraftStore";
-import { parseDiffRouteSearch, stripDiffSearchParams } from "../diffRouteSearch";
+import {
+  type DiffRouteSearch,
+  parseDiffRouteSearch,
+  stripDiffSearchParams,
+} from "../diffRouteSearch";
 import { useMediaQuery } from "../hooks/useMediaQuery";
 import { useStore } from "../store";
 import { Sheet, SheetPopup } from "../components/ui/sheet";
@@ -224,5 +228,8 @@ function ChatThreadRouteView() {
 
 export const Route = createFileRoute("/_chat/$threadId")({
   validateSearch: (search) => parseDiffRouteSearch(search),
+  search: {
+    middlewares: [retainSearchParams<DiffRouteSearch>(["diff"])],
+  },
   component: ChatThreadRouteView,
 });