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/i18n/locales/en/sidebar.json

@@ -16,6 +16,8 @@
   "hideWidget": "Hide widget",
   "expand": "Expand",
   "collapse": "Collapse",
+  "collapsePanel": "Collapse filters",
+  "expandPanel": "Expand filters",
   "any": "Any",
   "all": "All",
   "tagModeTooltip": "Match any vs all selected tags",