Grid density: a coarser board settles its tiles instead of stacking them

Stepping the grid coarser pulled tiles past the right edge back in and
left them wherever that put them — on top of one another, which the
board never allows under a drag. Tiles now settle in reading order: one
that would land on a tile already settled takes the first free spot, the
same rule a newly added tile follows.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -277,8 +277,10 @@ below. Nothing is installed without someone clicking.
   **Grid** stepper sets how many columns this browser cuts the board into —
   8 to 24, 12 by default. Finer lets tiles be smaller so more fit across (a
   4K wall), coarser suits a small screen read close up; row height follows,
-  so tiles keep their shape and simply change size. It saves with the
-  layout, so a named layout comes back at the density it was arranged on.
+  so tiles keep their shape and simply change size. Going coarser, a tile
+  that no longer fits where it was moves to the first free spot rather than
+  onto another tile. It saves with the layout, so a named layout comes back
+  at the density it was arranged on.
 - Every tile has a **status dot**: green = live, amber = connecting,
   red = upstream unreachable (hover it for details). Tiles reconnect
   automatically.