Add a sticky, focused search field to the file explorer #340

When Ctrl+B opens Files, show a sticky search field at the top with a visible caret and keyboard focus. Type to fuzzy-filter loaded directory entries by name/relative path, using ephemeral UI state only. Arrows select filtered results, Enter opens the selected match through the existing file-open path, Backspace/Delete edit the query, Ctrl+U clears it, Ctrl+B/Esc close, and directory changes clear the query. Match rendering, keyboard selection, wheel scrolling, and pointer hit-testing must use the same filtered list and fixed search-row geometry. No SQLite, recursive inventory, semantic retrieval, or Graft symbol scan is part of this slice. Stacked on #339 to retain the static title screen.