[web] Update threadActivityStore[activeThread].lastNavigatedTo state

Summary:
While the motivations for introducing `threadActivityStore` were about improving performance on `native`, we're making parallel changes to `web` for consistency.

---

Depends on D9402

Test Plan:
Navigated to various threads and checked `threadActivityStore` updated as expected (this is safe to land, will introduce migrations that clean things up when ripping out `lastNavigatedTo` and `lastPruned` from `messageStore.threads`):

{F795614}

{F795615}

Reviewers: ashoat, tomek, ginsu

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9404

web/redux/redux-setup.js

@@ -328,7 +328,7 @@ function validateState(oldState: AppState, state: AppState): AppState {
     oldActiveThread !== activeThread &&
     state.messageStore.threads[activeThread]
   ) {
-    // Update messageStore.threads[activeThread].lastNavigatedTo
+    const now = Date.now();
     state = {
       ...state,
       messageStore: {
@@ -337,10 +337,17 @@ function validateState(oldState: AppState, state: AppState): AppState {
           ...state.messageStore.threads,
           [activeThread]: {
             ...state.messageStore.threads[activeThread],
-            lastNavigatedTo: Date.now(),
+            lastNavigatedTo: now,
           },
         },
       },
+      threadActivityStore: {
+        ...state.threadActivityStore,
+        [activeThread]: {
+          ...state.threadActivityStore[activeThread],
+          lastNavigatedTo: now,
+        },
+      },
     };
   }