refactor(lib): move generic hooks, DnD engine, shortcut contract to lib

Continue M4 lib/ de-flattening with domain-agnostic infra:
- lib/hooks/: 9 pure generic hooks (useDebouncedValue, useIsMobile,
  useLongPressAction, useRangeSelection, useResizeClientHeight,
  useWindowVisibility, useSystemPrefersDark, useVirtualizerScrollMargin,
  useRemeasureGridVirtualizer) — all PURE (only react/zustand/tanstack deps).
- lib/dnd/DragDropContext.tsx: the generic mouse-event DnD engine (WebKitGTK
  HTML5-DnD workaround, ~24 importers, self-contained) — empties src/contexts/.
- lib/shortcuts/: shortcutActions + shortcutTypes (the action-id + binding
  contract; registry/dispatch/bindings stay app-level and import the contract,
  app->lib direction).

useWindowFullscreenState deliberately NOT moved — kept in hooks/ as app-shell
per the handoff iron-rule list (overrides its pure-helper appearance).

Pure move via deep @/lib/* specifiers. tsc 0, lint 0/0, full suite 319 files /
2353 tests green.

src/features/settings/components/BackdropSourceList.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useRef, useState } from 'react';
 import { ChevronDown, ChevronUp, GripVertical } from 'lucide-react';
-import { useDragDrop, useDragSource } from '@/contexts/DragDropContext';
+import { useDragDrop, useDragSource } from '@/lib/dnd/DragDropContext';
 import type { BackdropSource, BackdropSourcePref } from '@/cover/artistBackdrop';
 import type { BackdropSurface } from '@/store/themeStore';
 import { moveSourceTo, dropSourceBefore } from '@/features/settings/components/backdropReorder';

src/features/settings/components/InputTab.tsx

@@ -1,7 +1,7 @@
 import { useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import { Keyboard, RotateCcw, X } from 'lucide-react';
-import { IN_APP_SHORTCUT_ACTIONS, GLOBAL_SHORTCUT_ACTIONS } from '@/config/shortcutActions';
+import { IN_APP_SHORTCUT_ACTIONS, GLOBAL_SHORTCUT_ACTIONS } from '@/lib/shortcuts/shortcutActions';
 import { useGlobalShortcutsStore, type GlobalAction, buildGlobalShortcut, formatGlobalShortcut } from '@/store/globalShortcutsStore';
 import { useKeybindingsStore, type KeyAction, buildInAppBinding, formatBinding } from '@/store/keybindingsStore';
 import SettingsSubSection from '@/features/settings/components/SettingsSubSection';

src/features/settings/components/ReorderGripHandle.tsx

@@ -1,6 +1,6 @@
 import { useTranslation } from 'react-i18next';
 import { GripVertical } from 'lucide-react';
-import { useDragSource } from '@/contexts/DragDropContext';
+import { useDragSource } from '@/lib/dnd/DragDropContext';
 
 /**
  * Drag handle shared by the reorder customizers. Emits an id-based payload

src/features/settings/components/settingsSearch.ts

@@ -1,5 +1,5 @@
 import type { TFunction } from 'i18next';
-import { GLOBAL_SHORTCUT_ACTIONS, IN_APP_SHORTCUT_ACTIONS } from '@/config/shortcutActions';
+import { GLOBAL_SHORTCUT_ACTIONS, IN_APP_SHORTCUT_ACTIONS } from '@/lib/shortcuts/shortcutActions';
 import { SETTINGS_INDEX, matchScore, type Tab } from '@/features/settings/components/settingsTabs';
 
 export type SettingsSearchHit = {