Fix: limit unvisited feed items with native details collapse (closes #28)

[RealBhupesh]

What this PR does

This PR limits the default number of visible unvisited items per feed and collapses the remainder behind a native HTML5 <details> section.

Why

Some feeds can have 50+ unvisited items, which creates a very long page and makes scanning multiple feeds difficult.

Changes

• show first 20 unvisited items by default
• show remaining items under:
  • See other N unvisited
• keep existing behavior for:
  • marking one item as visited
  • favoriting items
  • “Mark all as visited” (applies to all unvisited items, not just visible ones)

Tests

• added e2e test for collapsing behavior:
  • verifies 20 visible by default
  • verifies summary count for hidden items
  • verifies expanding shows all items
• full e2e suite passes locally

Closes #28

[RealBhupesh]

Quick UX question: after expanding several “See other N unvisited” sections, getting back to the top is a bit tedious.
Would you accept a tiny follow-up PR for a “Back to top” button (or “Collapse all” action), keeping everything native/simple?

[strdr4605]

“Back to top”

Where should it navigate to? top of page?

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Please clean up unnecessary file changes in this PR. There should be only /e2e and /src file changes

[RealBhupesh]

Where should it navigate to? top of page?

something like that, if someone wants to scroll back to any page which sits on top or in the middle it would be hassle for him to scroll and scroll (if he has opened all the titles)

[strdr4605]

We need to add similar spacing, check if we can use flexbox, and gap. Also maybe reduce the spacing between details and first items, this way details are like a continuation of the list and users can scan it easier.