feat: restore last active channel on login

- Channel store saves {serverId, channelId} to localStorage on select
- ServerBar restores last server+channel after fetching servers
- Fetches channels for the saved server, then sets the channel if valid
- Gracefully handles missing/deleted servers or channels

web/src/stores/channel.ts

@@ -48,7 +48,19 @@ export const useChannelStore = create<ChannelState>((set) => ({
     }
   },
 
-  setActiveChannel: (id) => set({ activeChannelId: id }),
+  setActiveChannel: (id) => {
+    set({ activeChannelId: id });
+    if (id) {
+      // ponytail: persist last active channel for session restore
+      const state = useChannelStore.getState();
+      for (const [serverId, channels] of Object.entries(state.channelsByServer)) {
+        if (channels.some((c) => c.id === id)) {
+          localStorage.setItem('dumpster:lastChannel', JSON.stringify({ serverId, channelId: id }));
+          break;
+        }
+      }
+    }
+  },
 
   addChannel: (channel) =>
     set((state) => {