Place: a grid drag too small for its cells previews and places one frame

The place cursor's grid preview fell back to the raw drag rectangle when the
cells had no positive size, while the release still sent `place.grid`, which
rejected the area. Both now use the same cell calculation (`grid_cells`), and
a grid with no room for its cells shows and places the single proportional
frame of an ordinary drag.

docs/multi-image-placement.md

@@ -5,7 +5,7 @@ Choose **File > Place** (Cmd+D on macOS) and select several images. They load in
 - Click to place the next image at its natural size.
 - Drag to draw its frame: it keeps the image's proportions, and shows the scale beside the pointer.
 - Click an existing graphic frame to replace its contents.
-- While drawing, Right/Left add/remove columns and Up/Down add/remove rows. Release to place a grid in row order with 6-point gutters. Extra images remain in the cursor; unused grid cells create no empty frames.
+- While drawing, Right/Left add/remove columns and Up/Down add/remove rows. Release to place a grid in row order with 6-point gutters. Extra images remain in the cursor; unused grid cells create no empty frames. A drag too small for the grid's cells shows and places one frame instead, as an ordinary drag does.
 - Press Escape or select another tool to cancel the remaining placement.
 
 A single selected file retains the normal Place behavior, including filling a selected frame and PDF import options. Image batches support up to 1000 images. Rows and columns each range from 1 to 100. A batch containing an unreadable image fails without replacing the existing queue or changing the document. An invalid grid area also leaves the document and queue unchanged.