Keep the web client's saved channels and show who changed a mode (closes #176)

The web client saved its channel tabs to localStorage whenever the
tabs changed, including on page load, when only the Server tab
exists. That wrote an empty list over the saved one before login
read it, so a new session never rejoined anything. The save now
waits until the user is logged in, and login reads the saved list
before it marks the user logged in.

A MODE message from the server fell through to the Server tab as its
bare body. It now shows in the channel's tab as "alice sets mode
+v bob on #chan".

Model: opus-5-5

web/src/app.jsx

@@ -300,11 +300,14 @@ function App() {
     nickRef.current = nick;
   }, [nick]);
 
-  // Persist joined channels.
+  // Persist joined channels, once logged in: before then only the
+  // Server tab exists, and writing would erase the saved channels
+  // that onLogin rejoins.
   useEffect(() => {
+    if (!loggedIn) return;
     const channels = tabs.filter((t) => t.type === "channel").map((t) => t.name);
     localStorage.setItem("neoirc_channels", JSON.stringify(channels));
-  }, [tabs]);
+  }, [loggedIn, tabs]);
 
   // Clear unread on tab switch.
   useEffect(() => {
@@ -463,6 +466,15 @@ function App() {
 
           break;
         }
+        case "MODE": {
+          // The body is the change, with its value where it has one:
+          // ["+o bob"], ["+m"], ["+k", "secretpass"].
+          const modes = Array.isArray(msg.body) ? msg.body.join(" ") : "";
+          const text = `${msg.from} sets mode ${modes} on ${msg.to}`;
+          if (msg.to) addMessage(msg.to, { ...base, text, system: true });
+
+          break;
+        }
         case 353: {
           // RPL_NAMREPLY — each name is a hostmask (nick!user@host),
           // prefixed with @ for an operator or + for a voiced member.
@@ -759,6 +771,11 @@ function App() {
 
   const onLogin = useCallback(
     async (userNick, isResumed) => {
+      // Read before setLoggedIn(true): from then on the effect under
+      // "Persist joined channels" writes the open channel tabs over it.
+      const saved = JSON.parse(
+        localStorage.getItem("neoirc_channels") || "[]",
+      );
       setNick(userNick);
       setLoggedIn(true);
       addSystemMessage("Server", `Connected as ${userNick}`);
@@ -783,9 +800,6 @@ function App() {
       }
 
       // Fresh session — join any previously saved channels.
-      const saved = JSON.parse(
-        localStorage.getItem("neoirc_channels") || "[]",
-      );
       for (const ch of saved) {
         try {
           await api("/messages", {