Editor: Move Line and Copy Line keyboard shortcuts do nothing

[KrisPowers]

The Edit menu lists four line manipulation shortcuts that are no-ops:

• Alt+Up: Move Line Up
• Alt+Down: Move Line Down
• Shift+Alt+Up: Copy Line Up
• Shift+Alt+Down: Copy Line Down

These are standard shortcuts in VS Code and most modern editors, and developers rely on them constantly for quick reorganization without cut/paste.

What needs to happen

All four of these can be implemented purely in the frontend with basic buffer manipulation:

• Move Line Up / Down: Swap the current line (or all lines touched by the selection) with the line above/below it. Adjust cursor position to follow the moved content.
• Copy Line Up / Down: Insert a duplicate of the current line (or selected lines) immediately above or below. Cursor follows to the copy.
• If there is a multi-line selection, all lines in the selection should move or copy together as a unit.

Files likely involved

• app/frontend/src/components/GpuEditor.tsx -- where keybindings are handled and buffer mutations happen
• app/frontend/src/fullscreen/FullscreenIDE.tsx -- where the menu action currently short-circuits to a no-op

Acceptance criteria

• Alt+Up/Down moves the current line or selected block up or down by one line
• Shift+Alt+Up/Down inserts a duplicate of the current line or block above or below
• Cursor and selection move with the content
• Works with multi-line selections
• Undo reverts the operation in a single step