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.

test/noise-receive-test.el

@@ -208,3 +208,28 @@
      (should (string-match-p "Sarah> from sse" (buffer-string))))
    (let ((msg (car (noise-db-get-messages "+1"))))
      (should (string= "from sse" (gethash "body" msg))))))
+
+(ert-deftest noise-receive-sse-stores-then-open-shows-message ()
+  "An SSE event stores the message; opening the conversation later shows it."
+  (noise-receive-test--with-db
+   ;; No conversation buffer open — simulate SSE event arriving
+   (noise-receive--handle-sse-event
+    (noise-receive-test--ht
+     "envelope"
+     (noise-receive-test--ht
+      "sourceNumber" "+1" "sourceName" "Sarah"
+      "dataMessage" (noise-receive-test--ht
+                     "timestamp" 1717930000000
+                     "message" "hello from the void"))))
+   ;; Verify message is in DB
+   (let ((msgs (noise-db-get-messages "+1")))
+     (should (= 1 (length msgs)))
+     (should (string= "hello from the void" (gethash "body" (car msgs)))))
+   ;; Verify conversation has unread count
+   (let ((conv (noise-db-get-conversation "+1")))
+     (should (= 1 (gethash "unread_count" conv))))
+   ;; Now open the conversation and verify message renders
+   (require 'noise-conversation)
+   (with-current-buffer (noise-conversation-open "+1")
+     (should (string-match-p "Sarah> hello from the void" (buffer-string)))
+     (should (= 0 (gethash "unread_count" (noise-db-get-conversation "+1")))))))