fix: code-review round 1 — 4 findings

- og: guard safe_abs_path's None (missing/gc'd poster) before _jpeg_size, else the public /watch OG
  render crashed with an uncaught AttributeError
- channels: hoist the priority step button to module scope so it no longer remounts on every
  hold-repeat tick (which dropped the pointer capture and could let a hold run away on pointer drift)
- watch: only route audio through Web Audio on iOS (where a backgrounded <video> is suspended);
  other platforms keep their reliable native audio, avoiding a foreground-silence risk if the
  AudioContext can't resume
- share: drop the unstable `create` mutation object from the auto-create effect deps (it changes
  every render); the links.data trigger + autoTried ref already fire it exactly once

The clipboard-on-focus finding is intended (the user explicitly asked for auto-paste on focus; the
read is best-effort and browsers reject it without a gesture, so no real prompt fires).

backend/app/downloads/og.py

@@ -125,7 +125,10 @@ def _watch_tags(token: str, db: Session) -> str | None:
             tags.append(_tag("og:image:secure_url", image))
         if image.endswith("poster.jpg") and asset.poster_path:
             tags.append(_tag("og:image:type", "image/jpeg"))
-            dims = _jpeg_size(storage.safe_abs_path(settings.download_root, asset.poster_path))
+            # safe_abs_path returns None if the poster file is missing (gc'd) or escapes the root —
+            # guard it, else _jpeg_size(None) would raise an uncaught AttributeError on this public page.
+            poster_abs = storage.safe_abs_path(settings.download_root, asset.poster_path)
+            dims = _jpeg_size(poster_abs) if poster_abs else None
             if dims:
                 tags.append(_tag("og:image:width", str(dims[0])))
                 tags.append(_tag("og:image:height", str(dims[1])))