[Refactor] - Standardize design system tokens and enhance widget UI/UX

[InduwaraSMPN]

Summary

This PR implements a comprehensive overhaul of the widget's design system to improve visual consistency, maintainability, and user experience. The changes transition the codebase from hardcoded styles to a centralized, token-based architecture.

Key Changes:

• Design System Standardization:
  • Introduced component-tokens.ts to centralize logic for elevation (shadows), border-radius, and text tones.
  • Unified Flex, Stack, and Surface primitives to support shared layout props (padding presets, radius tokens, and elevation).
  • Expanded Text and Button components with new variants (chip, tab, inline, toolbar) and semantic sizes.
• UI/UX Enhancements:
  • Implemented staggered entrance animations for the HomeView and chat messages.
  • Added a CSS-based typing indicator for "thinking" states.
  • Improved micro-interactions, including scale transforms on button presses and backdrop blurs for headers.
  • Enhanced SuggestedActions with icons and improved hover/active states.
• Theming & Semantic Tokens:
  • Migrated notification styles and input borders to use CSS variables and semantic tokens, facilitating better support for High Contrast and Dark modes.
  • Updated color contrast logic for primary/accent colors.
• System & Build:
  • Widget Defaults: Updated index.html with fallback widget IDs and API hosts to ensure functional defaults when query parameters are absent.
  • Build Optimization: Adjusted vite.config.ts to disable code splitting, ensuring the widget is bundled as a single library file for easier integration.

Related issue

Resolves #design-system-consistency

Test plan

1. Load the widget without mtx-id params to verify fallback default settings.
2. Navigate between Home, Chat, and Help views to verify staggered entrance animations.
3. Trigger a "thinking" response from the agent to verify the new typing indicator animation.
4. Verify Button and IconButton states (hover, active, disabled) across different variants.
5. Check build output to ensure a single .js and .css bundle is generated without internal chunks.

Checklist

• CI passes (type-check + lint)
• Shared contracts in sync if changed (routes.ts, schema.ts, proto)