ui: align chat list and conversation buffer with mockups

Chat list (*signal*):
- Fringe ● now uses warning face (yellow), matching mockup's
  unread indicator color
- Modeline trailing dashes extended for visual balance

Conversation buffer (*signal:NAME*):
- Compose prompt > changed from bold to link face (blue),
  matching mockup's signal-blue prompt
- Attachment rendering: [attachment: ...] → [photo: ...]
- Modeline: (Signal Chat) → (Signal Chat Fill)
- Added header-line with key hints (RET send, M-RET newline,
  C-c C-g refresh), matching mockup's discoverability goal

test/noise-conversation-test.el

@@ -132,7 +132,7 @@
      (should (string= "draft text" (noise-conversation--current-input))))))
 
 (ert-deftest noise-conversation-format-attachment ()
-  "Attachment messages render the [attachment: ...] hint."
+  "Attachment messages render the [photo: ...] hint."
   (noise-conversation-test--with-db
    (noise-conversation-test--seed)
    (noise-db-insert-message "+1" :timestamp 1717930300.0
@@ -140,7 +140,7 @@
                             :has-attachment t
                             :attachment-path "IMG_2041.jpg")
    (with-current-buffer (noise-conversation-open "+1")
-     (should (string-match-p "\\[attachment: IMG_2041\\.jpg — RET to view\\]"
+     (should (string-match-p "\\[photo: IMG_2041\\.jpg — RET to view\\]"
                              (buffer-string))))))
 
 (ert-deftest noise-conversation-typing-indicator ()