feat(layout): full-height collapsible filter sidebar; sync status + scope relocated

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.

frontend/src/lib/storage.ts

@@ -20,6 +20,7 @@ export const LS = {
   statsTab: "siftlode.statsTab",
   adminUsersTab: "siftlode.adminUsersTab",
   navCollapsed: "siftlode.navCollapsed",
+  filterCollapsed: "siftlode.filterCollapsed",
   playlist: "siftlode.playlist",
   plSort: "siftlode.plSort",
   notifHistory: "siftlode.notifications",