Collapsible folders, long-press link menu, YouTube thumbnails, strip more Ars sequel chrome

- Feed list: folders now collapse (start collapsed, with a per-folder unread
  count so collapsing doesn't hide "is there anything new")
- Long-pressing a link inside an article shows an action menu (open in
  browser, share, save to Stash, save for later) instead of WebView's
  default text-selection popup -- confirmed live end to end, including the
  Stash/save-for-later paths creating a real standalone article
- New shared ensureStandaloneArticle() helper (data/ArticleSaving.kt),
  factored out of ShareReceiverActivity, now also used by the long-press menu
- YouTube iframe embeds become tap-to-open thumbnails (JS is off in the
  reader WebView, so the real player never worked anyway); tapping one goes
  straight to the YouTube app/browser instead of our own article-extraction
  pipeline
- Fixed back navigation through a chain of links-within-links: each hop was
  stacking a new backstack entry, so back stepped through the chain one link
  at a time instead of returning straight to the source article. Confirmed
  live: A -> link -> B -> link -> C, one back-press now goes straight to A
- Extended the <header> chrome-stripping from last round: also strip
  <footer>, an author bio card, and a "Most Read" trending-stories widget --
  confirmed live one of the latter showed up as a giant unrelated photo plus
  an empty numbered list (its lazy-loaded items never populate in this
  hidden, off-screen extraction WebView)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

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