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/miniPlayer/components/MiniPlayer.tsx

@@ -5,7 +5,7 @@ import { emit } from '@tauri-apps/api/event';
 import { invoke } from '@tauri-apps/api/core';
 import { useTranslation } from 'react-i18next';
 import { usePlayerStore } from '@/store/playerStore';
-import { registerQueueDragHitTest } from '@/contexts/DragDropContext';
+import { registerQueueDragHitTest } from '@/lib/dnd/DragDropContext';
 import MiniContextMenu from '@/features/miniPlayer/components/MiniContextMenu';
 import type { MiniSyncPayload, MiniControlAction, MiniTrackInfo } from '@/features/miniPlayer/utils/miniPlayerBridge';
 import {

src/features/miniPlayer/hooks/useMiniQueueDrag.ts

@@ -1,6 +1,6 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { emit } from '@tauri-apps/api/event';
-import { useDragDrop } from '@/contexts/DragDropContext';
+import { useDragDrop } from '@/lib/dnd/DragDropContext';
 import { usePlayerStore } from '@/store/playerStore';
 
 interface Args {

src/features/miniPlayer/hooks/useMiniSync.ts

@@ -1,6 +1,6 @@
 import { useEffect, useRef } from 'react';
 import { emit, listen } from '@tauri-apps/api/event';
-import { useWindowVisibility } from '@/hooks/useWindowVisibility';
+import { useWindowVisibility } from '@/lib/hooks/useWindowVisibility';
 import type { MiniSyncPayload } from '@/features/miniPlayer/utils/miniPlayerBridge';
 
 interface ProgressPayload {