Fix wrong-prefix silent ignore and Minimax image understand host

- Remove "Use .cmd for this role" reply when owner uses wrong prefix;
  wrong-prefix commands are now silently ignored for all users
- Fix AIImageUnderstandConfig default api_host to api.minimax.chat
  (minimax-coding-plan-mcp routes all calls there, not api.minimax.io)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/commands/dispatcher.py

@@ -80,10 +80,6 @@ class CommandDispatcher:
 
         expected_prefix = definition.expected_prefix(is_owner)
         if parsed.prefix != expected_prefix:
-            if is_owner:
-                await message.reply_text(
-                    f"Use {expected_prefix}{definition.name} for this account role"
-                )
             return True
 
         ctx = CommandContext(