Fix scroll regression and feed-setting reset on re-import

- ReaderScreen: the direction-ratio gesture rewrite (using a generic,
  non-oriented detectDragGestures to compute a custom dx/dy ratio) broke
  scrolling entirely -- reported live as "the article view doesn't scroll
  at all anymore." Root cause: detectDragGestures' touch-slop detection is
  isotropic (claims the first bit of movement in ANY direction before it
  knows which way the gesture is going), unlike detectHorizontalDragGestures
  which is orientation-aware and simply never engages for a vertical-
  dominant drag. Reverted to detectHorizontalDragGestures, keeping the
  distance-based (not edge-anchored) commit threshold from the previous
  fix. Verified live: a long article now scrolls properly on a diagonal
  swipe, and a deliberate horizontal swipe still changes pages.

- AddFeedScreen: both the OPML import and single-URL add built a bare
  FeedEntity(id, title, siteUrl) and upserted it (REPLACE) even when a
  feed with that id already existed -- silently wiping readerModeDisabled
  (and etag/lastModified/lastError) back to defaults on every re-import or
  re-add. This is exactly what made a feed's reader-mode toggle appear not
  to persist. Now preserves the existing row via .copy() when one exists.
  Verified live: set kottke.org's readerModeDisabled=1, re-imported the
  same OPML file (107 feeds), and confirmed the flag was still 1 afterward.

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