feat(fullscreen): multi-file selection in the file explorer

[KrisPowers]

Closes #73

Summary

• Ctrl/Cmd+Click toggles individual items in and out of the selection; Shift+Click selects a contiguous range anchored at the last-clicked item
• Multi-selected items render with an accent-tinted highlight, distinct from the gray active-file highlight
• Right-clicking a selection shows a batch context menu: Delete Selected (N) and Copy Paths
• Delete/Backspace with any selection active opens the confirmation dialog for all selected items; the dialog now accepts an array and says "Delete X Items" when multiple targets are involved
• Dragging from a selected item moves the entire selection to the drop folder in parallel, with ancestor/self-drop guards applied per-item
• Escape clears the selection

Test plan

• Ctrl+Click adds items one at a time; clicking again removes them; plain click clears the selection and opens the file
• Shift+Click selects the range between the anchor and the current item
• Multi-selected items show the accent background, not the active-file gray
• Right-click on a multi-selection shows Delete Selected / Copy Paths; right-click on a non-selected item shows the normal single-item menu
• Delete/Backspace triggers the batch confirmation dialog listing the count
• Dragging a selection moves all files; drag into a child folder of a selected folder is rejected
• Escape clears the selection without any other side effects