fix: restore window scrolling on main grid, optimize scroll-to-top, and enhance a11y & link security

[zarifnexus]

Summary of Changes

This PR resolves layout and scrolling constraints on the main card grid, streamlines the scroll-to-top behavior, and enhances link security and accessibility.

1. Restore Natural Window Scrolling

• Removed max-height: 70vh; and overflow-y: scroll; from .main-grid in public/global.css.
• Cards now flow naturally within the main viewport, eliminating awkward inner scrollbars and restoring standard window scrolling.

2. Optimize Scroll-to-Top Component

• Refactored src/scrollToTop.svelte to listen and scroll on window directly.
• Removed redundant and error-prone document.querySelector('.main-grid') queries.
• Added accessible attributes (role="button", tabindex="0", and aria-label="Back to top").

3. Security & A11y Improvements

• Added rel="noopener noreferrer" to all external outbound target="_blank" links in src/Category.svelte and src/Modal.svelte.
• Replaced aria-selected with aria-current on category buttons in src/Sidebar.svelte to fix the Svelte compiler a11y warning.

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Verification

• Built project with npm run build — 0 compiler errors and 0 warnings.
• Tested category navigation, modal view, deep link routing, and smooth window scrolling.

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