feat(web-ui): add a column picker and persist sort + columns per table - #584

Every VirtualTable-based table (downloads, shared, search, clients, ed2k
servers) now has a column picker to show/hide columns, and remembers its
sort direction and hidden columns per-table in localStorage.

- store.js: add loadPref/savePref, a guarded localStorage JSON get/set that
  degrades to the fallback when localStorage is disabled/full. Kept next to
  the in-memory pub/sub store rather than in a separate module.
- table.js: useTablePrefs hook persists sort + hidden columns under
  amule.table.<key>; ColumnPicker renders a native <details> checkbox menu
  for toggleable columns (those with a key and not flagged 'always').
- views: each table swaps its local sort state for useTablePrefs and drops a
  ColumnPicker into its toolbar; the full column set feeds sortRows while
  only the visible subset feeds VirtualTable. Checkbox + name columns are
  flagged 'always' (fixed); everything else, actions included, is toggleable.
  Columns that lacked a key (server version/address/priority, shared
  priority) gain one so they sort and hide too.
- split-detail.js: use the shared loadPref/savePref for the panel height
  instead of a bespoke unguarded localStorage access.
- app.css: styles for the .col-picker dropdown, with a scrollable menu and
  roomier tap targets on touch screens.
- i18n (en/es): add the 'Columns' key.