feat(pan): interactive PAN mode with left-drag (no middle button needed)

The PAN command/tool dispatched a command nobody handled, so it did
nothing. Combined with no middle mouse button on a trackpad/web client,
there was no way to pan at all (issue 164).

PAN (and the "P" alias) now enters a pan mode: a left-button drag pans the
view, reusing the existing middle-button pan path. Exit with Esc or by
starting another command. While active the crosshair is hidden and the
cursor becomes a hand (open when hovering, closed while dragging). PAN/P
are also added to command-line autocomplete.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -263,6 +263,7 @@ impl OpenCADStudio {
                 tab.last_cursor_screen,
                 !is_paper && self.show_viewcube,
                 tile_edges,
+                tab.pan_mode,
             )
         };