Sidebar: no multi-file selection in the file tree

[KrisPowers]

You cannot select more than one file at a time in the sidebar file tree. There is no Ctrl+Click or Shift+Click to build a selection, and no way to perform batch operations (delete multiple files, move a group of files to a different folder, etc.).

By contrast, the tab bar already supports multi-select (Ctrl/Cmd+Click, Shift+Click), so the pattern exists in the codebase, it just has not been brought into the file explorer.

What needs to happen

• Ctrl+Click (or Cmd+Click on Mac) adds/removes a file from the current selection without clearing other selected items
• Shift+Click selects a contiguous range from the last-clicked item to the current one
• Selected files get a visual highlight distinct from the single "active" highlight
• Right-clicking while multiple files are selected shows a context menu with batch actions: Delete Selected, Move Selected (drag or context menu), Copy Path(s)

Files likely involved

• app/frontend/src/fullscreen/FileExplorer.tsx -- the main file tree component; selection state lives here
• app/frontend/src/fullscreen/ContextMenu.tsx -- context menu needs new batch-action entries

Acceptance criteria

• Ctrl+Click adds files to selection, Shift+Click selects a range
• Visually selected items are clearly distinguishable from the active (open) item
• Right-clicking a selection shows batch actions: Delete Selected, Copy Paths
• Pressing Delete or Backspace with a multi-selection triggers the confirmation dialog for all selected files
• Dragging a selection moves all selected files to the drop target folder