feat(sidebar): §14 sidebar orchestration, filter chips, pinned rail, trailing column (#850)

* feat(sidebar): §14 sidebar orchestration (filter chips, pinned rail, trailing column)

- Add All / Up / Down / Updates filter chips with live counts; active chip
  filters the list; chip-filtered files computed in EditorLayout with useMemo
- Surface the PINNED group with a 3px cyan left rail and glow via the
  sidebarPinnedGroupRail token; reuses the brand token already on the active row
- Compact brand row from three stacked elements to a single 44px horizontal bar
- Restructure StackRow trailing column as fixed slots: label dots (max 3 + +N
  overflow), update-dot | git-pending icon (priority order), kebab; add reserved
  invisible checkbox slot for PR2 bulk mode
- Export statusText / statusColor from StackRow and reuse them in StackList
  remote-results section to remove the duplicate inline logic
- Lift filterChip state and chip-filtered files to EditorLayout; remove
  filterChip from StackListProps to eliminate the dual-path redundancy
- Remove unused labels param from buildGroups and the void labels workaround
- Wrap filteredFiles in useMemo so filterCounts memo is not defeated on every
  render

* fix(sidebar): move statusText and statusColor to stack-status-utils

react-refresh/only-export-components requires component files to export
only components. Move the two utility functions and StackRowStatus type
to a dedicated stack-status-utils.ts so StackRow.tsx is a pure component
module. Update StackList.tsx and EditorLayout.tsx to import from the new
source directly.

frontend/src/components/sidebar/sidebar-styles.ts

@@ -19,3 +19,12 @@ export const sidebarGroupHeader = cn(
   'text-[9px] leading-3 tracking-[0.22em] uppercase text-stat-subtitle',
   'cursor-pointer select-none',
 );
+
+export const sidebarRowCheckboxSlot = 'shrink-0 w-4 h-4 opacity-0 pointer-events-none flex-shrink-0';
+
+export const sidebarPinnedGroupRail = cn(
+  'relative before:absolute before:left-0 before:top-0 before:bottom-0',
+  'before:w-[3px] before:rounded-sm before:bg-brand',
+  'before:shadow-[0_0_6px_var(--brand)]',
+  'pl-[13px]',
+);