Videos: know what a clip costs before fetching it; vertical clips only

- What a clip costs is learned up front: the post's imeta (size, dim,
  image) when it says, else one 64 KB ranged request — the total from
  Content-Range, whether ranges are served, whether the MP4's index
  (moov) comes before its data, and the picture size from the track
  header. Up to three at a time; remembered across sessions. It also
  replaces the player-based shape probe the feed ran on every clip.
- Clips over 25 MB (8 MB on 2G/3G, 3 MB with data saver), above 1080p,
  index-last files over 8 MB from servers without ranges, or clips that
  couldn't be measured wait for a tap: a cover saying what they cost
  ("50 MB", "4K"). Settings → Nostr → Videos: Autoplay large videos.
- The pager loads only the clip on screen in full; the next gets its
  opening if within budget; everything else holds nothing. Sizes are
  asked three slides ahead.
- Feed clips fetch nothing until they're about to play (preload none),
  with the post's still as a poster.
- Video grid tiles show a first frame only for small clips that start
  quickly; others show their cost.
- Videos lists only clips known to stand taller than wide; one of unknown
  shape is measured and joins when it qualifies; choosing Videos with none
  known yet opens the pager on the first that turns up.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ULQZS9MQa6iAsXnrJUD9Du

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