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

noise-chat-list.el

@@ -66,7 +66,7 @@
                                           conversations)
                                   :initial-value 0)))
     (setq mode-line-format
-          (format " ⌁ %s   %d chats %s %s   (%s) ─ U:%%- ─ Top ───"
+          (format " ⌁ %s   %d chats %s %s   (%s) ─ U:%%- ─ Top ────────────"
                   (propertize "*signal*" 'face 'bold)
                   total
                   (if (> unread-total 0)
@@ -109,7 +109,7 @@ CONV is a hash table with string keys from noise-db."
            (t (concat sender ": " preview))))
          (preview-str (truncate-string-to-width preview-prefix 36 nil nil "…"))
          (time-str (noise-chat-list--format-time timestamp)))
-    (concat " " fringe " " unread-str " "
+    (concat " " (propertize fringe 'face 'warning) " " unread-str " "
             (propertize (format "%-18s" name-str) 'face 'bold) "  "
             preview-str "  "
             (propertize time-str 'face 'shadow))))