Fix dropdown selection display on messages page

Added prompt option "Select a recipient..." to the To dropdown on
the messages page. This provides a clear placeholder and makes the
selected user's name visible after selection.

Before: Dropdown showed no placeholder, making it unclear what was selected
After: Shows "Select a recipient..." by default, then displays the
       selected user's name when a recipient is chosen

Fixes the issue where clicking a user in the dropdown didn't show
their name in the dropdown field.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/views/messages/index.html.erb

@@ -108,7 +108,7 @@
               </label>
               <%= f.select(:receiver_id,
                           options_from_collection_for_select(User.all, :id, :full_name),
-                          {},
+                          { prompt: "Select a recipient..." },
                           { class: "form-select form-select-lg" }) %>
               <small class="text-muted">Select message recipient</small>
             </div>