feat(sidebar): bulk stack operations (#854)

* feat(sidebar): bulk stack operations (select, start/stop/restart/update)

- Add ⊞ bulk mode toggle in SidebarActions (cyan active state, tooltip "Bulk
  mode (B)"); keyboard shortcut B toggles, Esc exits, Ctrl+A selects all
  visible (chip-filtered) stacks
- Reserved checkbox column in StackRow becomes visible and interactive in bulk
  mode; clicking a row in bulk mode toggles selection instead of opening the
  stack; kebab and context-menu still work in either mode
- SidebarBulkBar appears below filter chips when >=1 stack selected: shows
  count, Start / Stop / Restart / Update actions; Update is disabled with a
  Skipper TierBadge for Community licenses
- useBulkStackActions hook fans out operations via Promise.allSettled and
  surfaces an aggregate toast ("3 of 4 restarted; 1 failed: plex")
- Bulk update enforced Skipper-gated frontend-side (isPaid check in hook) and
  sends x-bulk-mode header for backend defense-in-depth
- Extract isInputFocused / isPaletteOpen to lib/keyboard-guards.ts; both
  useStackKeyboardShortcuts and the new bulk keyboard effect now share the
  same guards instead of duplicating the logic
- chipFilteredFiles captured via useRef in bulk keyboard effect so the listener
  is not torn down and re-added on every status-poll cycle

* fix(sidebar): separate TooltipProviders for bulk and scan icon buttons

Wrapping both icon buttons in a single TooltipProvider made them
render as one flex child, collapsing the gap-2 between them.
Splitting into two independent TooltipProviders restores the 8px
gap and right padding of the scan button.

frontend/src/hooks/useStackKeyboardShortcuts.ts

@@ -1,16 +1,6 @@
 import { useEffect, useRef } from 'react';
 import type { StackMenuCtx } from '@/components/sidebar/sidebar-types';
-
-function isInputFocused(): boolean {
-  const el = document.activeElement as HTMLElement | null;
-  if (!el) return false;
-  const tag = el.tagName;
-  return tag === 'INPUT' || tag === 'TEXTAREA' || el.isContentEditable;
-}
-
-function isPaletteOpen(): boolean {
-  return !!document.querySelector('[role="dialog"] [cmdk-root]');
-}
+import { isInputFocused, isPaletteOpen } from '@/lib/keyboard-guards';
 
 export function useStackKeyboardShortcuts(
   selectedFile: string | null,