fix: show notifications and unread count for SSE messages arriving when chat closed

Three gaps caused messages received via SSE to be invisible when
the conversation buffer wasn't already open:

1. Echo-area notification (noise-receive.el): when an SSE message
   arrives and the conversation buffer isn't in a visible window,
   show "[Noise] <name>" in the echo area.

2. Modeline unread counter (noise.el): the noise-mode lighter now
   shows Noise[N] for N unread messages via a lightweight SUM
   aggregation across all conversations.

3. Auto-refresh hook (noise-conversation.el): a
   window-buffer-change-functions hook re-renders *signal:* buffers
   when they become visible, covering direct C-x b switches.

Also adds regression test:
noise-receive-sse-stores-then-open-shows-message —
store via SSE handler with no buffer open, then open and verify.

noise-receive.el

@@ -44,13 +44,22 @@ Returns the number of new messages stored."
       (message "Received %d new message%s" new (if (= new 1) "" "s")))
     new))
 
+(defun noise-receive--announce-message (conv-id)
+  "Show a brief echo-area notification for a new message in CONV-ID."
+  (let* ((conv (noise-db-get-conversation conv-id))
+         (name (if conv (or (gethash "name" conv) conv-id) conv-id)))
+    (message "[Noise] %s" name)))
+
 (defun noise-receive--handle-sse-event (event)
   "Handle an SSE EVENT from signal-cli.
 Processes the envelope and updates UI."
   (noise-db-init)
   (when-let ((envelope (noise-receive--field event "envelope")))
     (when-let ((conv-id (noise-receive--process-envelope envelope)))
-      (noise-conversation-refresh-for conv-id)
+      (let ((buf (noise-conversation--buffer-for conv-id)))
+        (if (and buf (get-buffer-window buf t))
+            (noise-conversation-refresh-for conv-id)
+          (noise-receive--announce-message conv-id)))
       (when-let ((buf (get-buffer "*signal*")))
         (with-current-buffer buf
           (noise-chat-list--render))))))