refactor(ux): address /code-review high findings on the fixpack

- Extract the optimistic + debounced-flush feed-priority flow into a shared
  useChannelPriority hook (mirrors useChannelTagActions); the manager and the
  channel-detail page now share it instead of two drifting copies.
- NavSidebar: drop the always-true `slim` const and its dead expanded-rail
  branches; refresh the now-stale pinned/collapsible comments.
- Remove the orphaned `navCollapsed` localStorage key (no readers/writers left
  after the always-slim rail).
- Channel page: drop the wrapper native `title` that duplicated PriorityStepper's
  own Tooltip.

The keyboard-focus nav-label finding needed no change: the popover platform
already wires useFocus (visibleOnly), so a real Tab press shows the flyout label
(verified in-browser).

frontend/src/lib/storage.ts

@@ -39,7 +39,6 @@ export const LS = {
   auditTable: "siftlode.auditTable",
   statsTab: "siftlode.statsTab",
   adminUsersTab: "siftlode.adminUsersTab",
-  navCollapsed: "siftlode.navCollapsed",
   filterCollapsed: "siftlode.filterCollapsed",
   playlist: "siftlode.playlist",
   plSort: "siftlode.plSort",