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/nowPlaying/components/NpCardWrap.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { useDragSource } from '@/contexts/DragDropContext';
+import { useDragSource } from '@/lib/dnd/DragDropContext';
 import type { NpCardId } from '@/features/nowPlaying/store/nowPlayingLayoutStore';
 
 interface NpCardWrapProps {

src/features/nowPlaying/pages/NowPlaying.tsx

@@ -13,7 +13,7 @@ import { useAuthStore } from '@/store/authStore';
 import { useLyricsStore } from '@/store/lyricsStore';
 import { songToTrack } from '@/utils/playback/songToTrack';
 import { useRadioMetadata } from '@/features/radio';
-import { useDragDrop } from '@/contexts/DragDropContext';
+import { useDragDrop } from '@/lib/dnd/DragDropContext';
 import OverlayScrollArea from '@/ui/OverlayScrollArea';
 import {
   useNpLayoutStore, NP_CARD_IDS,