feat(theme): add support for light theme and mode selector

[DibashSarkar]

Description

This PR introduces support for Light Mode across the application, adding a theme selector to allow users to switch between Light, Dark, and System preferences.

Note: This is part 1 of splitting up the previous monolithic PR to allow for easier, safer reviews.

Changes

• Configured Tailwind CSS to support dark: variants properly alongside light mode defaults.
• Updated globals.css with a complete set of light and dark CSS variables for the color palette.
• Added a ThemeToggle component to the user settings/header for seamless switching.
• Ensured all existing components (buttons, cards, text, and inputs) have appropriate color contrast and styling in both themes.
• Replaced hardcoded color values with semantic Tailwind classes (e.g., bg-background, text-foreground).

Why this is needed

To improve accessibility and provide a better user experience for those who prefer working in light mode environments.

Testing

• Verified manual switching between Dark and Light mode.
• Verified System mode respects OS-level theme preferences.
• Tested across major views (dashboard, settings, webhooks) to ensure no visual regressions or illegible text.