redesign signin & loading view

[HALQME]

This pull request introduces several UI improvements and code refactoring for the sign-in flow and profile components. The main changes include extracting a new LoadingView component for a cleaner sign-in experience, updating icon usage for consistency, improving the display of usernames, and minor backend logic adjustments.

Sign-in Flow Refactor and UI Enhancement:

• Extracted a new LoadingView component for the sign-in redirect/loading state, simplifying SignIn.vue and improving code reusability. The new component provides a spinner and a fallback link if redirection fails (LoadingView.vue, SignIn.vue) [1] [2] [3].
• Removed the progress bar and refactored the loading state logic in SignIn.vue for a cleaner UI and better separation of concerns (SignIn.vue) [1] [2].

Icon and Visual Improvements:

• Replaced usage of imported icon components with Lucide icon classes (e.g., i-lucide:chevrons-up-down, i-lucide:check) in ProfileCard.vue for grade/year dropdowns, improving icon consistency and reducing bundle size (ProfileCard.vue) [1] [2] [3] [4].
• Updated styling for selected dropdown options to highlight them in blue and improved layout for better accessibility and clarity (ProfileCard.vue) [1] [2].

Profile Display Improvements:

• Updated the username display in UserHeader.vue to always show an @ prefix if a username exists, improving clarity and consistency (UserHeader.vue).

Backend Adjustment:

• Modified the user update handler to explicitly set username to an empty string if not provided, ensuring correct backend behavior when clearing the username (clerk.ts).