Show the direct-message web tab even when other tabs change first (closes #202)

openDM took the direct-message tab's position from the tab list of the
render its caller came from. /msg calls it only after the PRIVMSG
request returns. By then the server's copy of the message, or a join or
part from another client of the session, may have changed the tabs. The
active tab then became another tab or none, and the next typed line went
to the wrong place or was dropped.

openDM now finds the tab by name inside its setTabs update and makes it
active there, as joinChannel does.

Model: opus-5-5

web/src/app.jsx

@@ -948,16 +948,23 @@ function App() {
 
   const openDM = (targetNick) => {
     if (targetNick === nickRef.current) return;
+    // Show the direct-message tab. Preact runs this update at once on
+    // the newest tab list, so the tab is found there by name: the
+    // server's copy of the user's message may already have added it,
+    // and other tabs may have opened or closed.
     setTabs((prev) => {
-      if (prev.find((t) => t.type === "dm" && t.name === targetNick))
+      const idx = prev.findIndex(
+        (t) => t.type === "dm" && t.name === targetNick,
+      );
+      if (idx >= 0) {
+        setActiveTab(idx);
+
         return prev;
+      }
+      setActiveTab(prev.length);
 
       return [...prev, { type: "dm", name: targetNick }];
     });
-    const idx = tabs.findIndex(
-      (t) => t.type === "dm" && t.name === targetNick,
-    );
-    setActiveTab(idx >= 0 ? idx : tabs.length);
   };
 
   const handleCommand = async (text) => {