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.el

@@ -76,11 +76,27 @@ Queries the daemon for registered accounts and sets `noise-account'."
     (noise-rpc-error
      (message "Error: %s" (cdr err)))))
 
+(defun noise--unread-lighter ()
+  "Return the mode-line lighter string with unread count."
+  (let ((count 0))
+    (when (and (boundp 'noise-db--connection)
+               noise-db--connection
+               (fboundp 'sqlitep)
+               (sqlitep noise-db--connection))
+      (condition-case nil
+          (setq count (or (caar (sqlite-select noise-db--connection
+                                                "SELECT COALESCE(SUM(unread_count), 0) FROM conversations"))
+                          0))
+        (error 0)))
+    (if (> count 0)
+        (format " Noise[%d]" count)
+      " Noise")))
+
 ;;;###autoload
 (define-minor-mode noise-mode
   "Toggle Noise Signal client mode."
   :global t
-  :lighter " Noise"
+  :lighter (:eval (noise--unread-lighter))
   :group 'noise
   (if noise-mode
       (progn
@@ -88,6 +104,5 @@ Queries the daemon for registered accounts and sets `noise-account'."
         (noise-receive-start-sse))
     (noise-receive-stop-sse)
     (message "Noise mode disabled")))
-
 (provide 'noise)
 ;;; noise.el ends here