feat(frontend): move/copy line shortcuts (Alt+Up/Down, Shift+Alt+Up/Down)

[KrisPowers]

Summary

• Implements Alt+Up / Alt+Down to move the current line (or selected block) up or down by swapping it with the adjacent line
• Implements Shift+Alt+Up / Shift+Alt+Down to duplicate the current line or selected block above or below
• All four operations work with multi-line selections, treating the entire touched line range as a unit
• Cursor and selection anchor follow the moved/copied content in all cases
• Each operation is a single backend edit call, so undo reverts it in one step
• Wired up both as keyboard shortcuts in GpuEditor and as menu actions via the existing MenuBar command IDs

Closes #70

Test plan

• Open a file, place cursor on any line
• Alt+Up/Down: line swaps with the line above/below; cursor follows
• Shift+Alt+Up: duplicate inserted above, cursor on original (now one line lower)
• Shift+Alt+Down: duplicate inserted below, cursor stays on original
• Select multiple lines and repeat all four; entire block moves/copies as a unit
• Undo after each operation reverts in a single step
• Edit > Move Line Up / Move Line Down / Copy Line Up / Copy Line Down menu items work