ja: implement public share landing page and improve error mapping

- Add web/share.html with a minimal HTML5 video player, theme variables,
  centered layout, play overlay, and Back to Home link.
- Update handleSharePage to return HTML for browser Accept headers
  (text/html or empty) and JSON for application/json. Return 410 Gone
  for expired shares.
- Update handleShareStream to map service errors to distinct HTTP codes:
  ErrShareExpired -> 410, ErrShareNotFound/ErrMediaNotFound -> 404.
- Improve ValidateShareToken and StreamSharedMedia to return sentinel
  errors (ErrShareNotFound, ErrShareExpired, ErrMediaNotFound) instead
  of generic string errors.
- Update and add tests for share page HTML/JSON negotiation and share
  stream 404/410 responses.

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