UI/UX: Standardize input component icons for dark mode compatibility

Description:
Currently, the chatbot's Input component and file attachment previews rely on hardcoded emojis (📎, 🖼️, 📄) and hardcoded hex colors (e.g., #fff, #ccc, #f8f9fa). Because these values don't respect Jenkins' built-in theming, the input area and attachment chips are glaringly bright and difficult to read when switching to dark mode. Additionally, the emojis do not visually align with the lucide-react icons already being used in the chatbot header.

Proposed Solution:

• Replace all hardcoded input and attachment emojis with lucide-react icons (Paperclip, Image, File, Send, CircleStop) to ensure UI consistency across the plugin.
• Refactor the chatbotStyles to replace hardcoded hex colors in the input area and attachment containers with standard Jenkins CSS variables (e.g., var(--button-background), var(--text-color), var(--panel-background)).
• Update the flexbox styling to ensure the new icons are perfectly centered inside their respective buttons and attachment chips.

Expected Behavior:
The input field, action buttons (Send/Cancel/Attach), and file attachment preview chips should seamlessly adapt to both light and dark modes, matching the exact styling behavior of the header.