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-conversation.el

@@ -37,10 +37,17 @@
     map)
   "Keymap for `noise-conversation-mode'.")
 
+(defconst noise-conversation--mode-header
+  (concat (propertize "RET" 'face 'help-key-binding) " send   "
+          (propertize "M-RET" 'face 'help-key-binding) " newline   "
+          (propertize "C-c C-g" 'face 'help-key-binding) " refresh")
+  "Header line showing key hints for `noise-conversation-mode'.")
+
 (define-derived-mode noise-conversation-mode text-mode "Signal Chat"
   "Major mode for a Noise Signal conversation buffer.
 \\{noise-conversation-mode-map}"
-  :group 'noise)
+  :group 'noise
+  (setq-local header-line-format noise-conversation--mode-header))
 
 ;;;###autoload
 (defun noise-conversation-open (conversation-id)
@@ -107,7 +114,7 @@ Preserves any input already typed at the prompt."
             (propertize (make-string noise-conversation--width ?─)
                         'face 'shadow)
             "\n"
-            (propertize "> " 'face 'bold))
+            (propertize "> " 'face 'link))
     (add-text-properties (point-min) (point)
                          '(read-only t
                            front-sticky (read-only)
@@ -149,7 +156,7 @@ Preserves any input already typed at the prompt."
                    (or (gethash "attachment_path" msg) ""))))
         (setq body (concat body (if (string-empty-p body) "" " ")
                            (propertize
-                            (format "[attachment: %s — RET to view]" file)
+                            (format "[photo: %s — RET to view]" file)
                             'face 'link)))))
     (let ((line (concat (propertize time-str 'face 'shadow) " "
                         (propertize name 'face 'bold)
@@ -167,13 +174,13 @@ Preserves any input already typed at the prompt."
 (defun noise-conversation--update-modeline ()
   "Update the mode line with the E2E indicator and typing status."
   (setq mode-line-format
-        (format " %s %s   %s%s ─ U:%%- ─ Bot ───"
+        (format " %s %s   %s%s ─ U:%%- ─ Bot ───────────"
                 (propertize "⌁ E2E" 'face 'success)
                 (propertize (buffer-name) 'face 'bold)
                 (if noise-conversation--typing
                     (format "%s is typing…   " noise-conversation--typing)
                   "")
-                "(Signal Chat)"))
+                "(Signal Chat Fill)"))
   (force-mode-line-update))
 
 ;;; Composing and sending