Restructure the app shell into three top-level columns:
- The per-user sync status (video counts + live sync state) moves from the top bar to a
compact block at the top of the left nav rail (icon-only with a tooltip when collapsed).
- The feed's Mine/Library scope toggle moves to the top of the filter sidebar.
- The filter sidebar becomes a full-height sibling column with its own collapse control
(a thin rail carrying the active-filter count), mirroring the nav rail. The top bar is
now just the feed search / page title.
- Both panels' collapsed state is persisted to the user's preferences (server-side, so it
follows the account across devices), seeded from a localStorage cache to avoid a flash.
Default: both panels open.
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