Add Repeat Video Fixer; flip placeholder default; apply review fixes
Counts feed-card viewport sightings (>=50% visible, once per video per
SPA navigation) via IntersectionObserver, batched to the background and
stored in storage.local.seenCounts with last-seen pruning. Cards seen at
least repeatThreshold times before (default 1, configurable 1-10) are
hidden with the same hard-hide/placeholder modes as the Watch Filter;
repeat placeholders offer a "Show anyway" per-video count reset.

Hide decisions use sighting counts snapshotted at navigation time so a
card never disappears while it is being looked at; the snapshot only
syncs downward mid-page (resets/clears).

placeholderMode now defaults to true for new installs.