fix: next and prev in the article view follow the filtered list

[thebanri]

Following up on #201: this is one of the fixes from it, on Bubble Tea.

With a filter applied, l/h in the article view open the wrong article. list.Index() counts the filtered rows, but next and prev indexed list.Items(), which is every row. Filter to two articles with another one between them in the full list, and next opens the one in between.

Switching to VisibleItems() isn't enough on its own. Opening an article calls UpdateList, and a filtered list then refilters in the background. The FilterMatchesMsg arrived while the article view was showing, so it went to the viewport and was dropped. In the article view the list had no visible rows, and next and prev did nothing. Update now hands FilterMatchesMsg to the list whichever view is showing.

Before / after, with three articles where the filter golang shows two:

open "golang two", press l   master: "rust news"   this PR: "golang one"
then press h                 master: "golang two"  this PR: "golang two"

Checked by running nom in a pty against a local feed, and with TestNextAndPrevFollowTheFilteredList. The test drives the model through Update the way the program does: it types the filter, opens an article, presses l and h, and runs the commands they return. It fails on master, and it also fails with only the VisibleItems() half of the change.

Not changed here: m (mark read) in the article view still takes its undo backup from list.Items() by the filtered index. It has the same mix-up, but fixing it touches bubbles' RemoveItem/InsertItem, which are themselves unfiltered-index based, so I left it for a separate change.

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