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-conversation.el

@@ -245,5 +245,19 @@ NAME is the contact typing; ACTION is signal-cli's STARTED or STOPPED."
     (with-current-buffer buf
       (noise-chat-list--render))))
 
+;;; Auto-refresh when conversation buffers become visible
+
+(defun noise-conversation--on-buffer-visible (_frame)
+  "Refresh any stale `*signal:*' conversation buffer that just became visible."
+  (dolist (window (window-list nil 'never (selected-frame)))
+    (when-let ((buf (window-buffer window)))
+      (when (and (string-prefix-p "*signal:" (buffer-name buf))
+                 (not (string= "*signal*" (buffer-name buf))))
+        (with-current-buffer buf
+          (when (derived-mode-p 'noise-conversation-mode)
+            (noise-conversation--render)))))))
+
+(add-hook 'window-buffer-change-functions #'noise-conversation--on-buffer-visible)
+
 (provide 'noise-conversation)
 ;;; noise-conversation.el ends here