Added the BridgeWise theming API and component-level customization

[Kaylahray]

Added the BridgeWise theming API and component-level customization. closes #56

Summary

• Introduced a high-level BridgeWiseTheme interface and a BridgeWiseProvider wrapper on top of the existing token-based theme system to simplify SDK theming for host dApps.
• Wired the provider into the existing ThemeProvider/CSS variable pipeline, supporting light/dark/system modes with defaultTheme + darkTheme fallbacks and --bw-* CSS custom properties.
• Added className and style overrides to public components (TransactionHeartbeat, BridgeHistory, BridgeCompare) to allow per-instance layout and style customization without forking.
• Updated the demo app to showcase custom theming and dark mode, and extended the @bridgewise/ui-components README with a “Theming Guide”.
• Added unit tests validating theme merging behavior and CSS variable generation.