fix(web): unify mic/send button and restore scroll-down

- show one action button in composer: mic when empty, send when text exists

- add floating scroll-to-bottom button in message list

- exclude non-text/media messages from Chat Info links list to avoid duplicates

web/src/components/MessageComposer.tsx

@@ -48,6 +48,7 @@ export function MessageComposer() {
   const [recordingState, setRecordingState] = useState<RecordingState>("idle");
   const [recordSeconds, setRecordSeconds] = useState(0);
   const [dragHint, setDragHint] = useState<"idle" | "lock" | "cancel">("idle");
+  const hasTextToSend = text.trim().length > 0;
 
   useEffect(() => {
     recordingStateRef.current = recordingState;
@@ -560,23 +561,27 @@ export function MessageComposer() {
           }}
         />
 
-        <button
-          className="h-[42px] w-[72px] rounded-full bg-sky-500 px-4 text-sm font-semibold text-slate-950 hover:bg-sky-400 disabled:opacity-60"
-          disabled={recordingState !== "idle"}
-          onClick={handleSend}
-          type="button"
-        >
-          Send
-        </button>
-
-        <button
-          className={`h-[42px] w-[56px] rounded-full px-3 text-sm font-semibold ${recordingState === "idle" ? "bg-emerald-500 text-slate-950 hover:bg-emerald-400" : "bg-slate-700 text-slate-300"}`}
-          disabled={isUploading}
-          onPointerDown={onMicPointerDown}
-          type="button"
-        >
-          Mic
-        </button>
+        {hasTextToSend ? (
+          <button
+            className="h-[42px] w-[56px] rounded-full bg-sky-500 px-3 text-sm font-semibold text-slate-950 hover:bg-sky-400 disabled:opacity-60"
+            disabled={recordingState !== "idle" || !activeChatId}
+            onClick={handleSend}
+            type="button"
+          >
+            ➤
+          </button>
+        ) : (
+          <button
+            className={`h-[42px] w-[56px] rounded-full px-3 text-sm font-semibold ${
+              recordingState === "idle" ? "bg-emerald-500 text-slate-950 hover:bg-emerald-400" : "bg-slate-700 text-slate-300"
+            }`}
+            disabled={isUploading || !activeChatId}
+            onPointerDown={onMicPointerDown}
+            type="button"
+          >
+            🎤
+          </button>
+        )}
       </div>
 
       {selectedFile ? (