Grid density: cut the board finer or coarser, per browser

Twelve columns fit a booth monitor and nothing else well: on a 4K wall a
three-wide tile is nearly a thousand pixels, and on a small screen four tiles
across is the most the grid allows. A stepper in the Layouts menu now cuts
the board into 8, 10, 12, 16, 20 or 24 columns. Row height follows the
column width, so cells keep their shape and a 3×2 tile is still a 3×2 tile,
only smaller — finer lets tiles be smaller so more fit, coarser suits a
screen read close up.

It is this browser's choice, not the server's: two displays at one location
can want different densities, so it saves with the layout — the local one
and a named one on the server — and a named layout comes back at the density
it was arranged on, which the load message says. An older layout with no
density is the default twelve.

Tiles keep their grid units when the density changes; the board simply gets
denser or coarser around them, and a tile now past the right edge is pulled
in. A module's minSize is enforced in cells as declared, so on a finer grid a
tile can be smaller than it could at twelve, which is the point; a new tile's
defaultSize is scaled to the density so it lands at about the size its author
meant. Leaving fullscreen goes back to the chosen density rather than the
stylesheet's twelve — that reset predates any notion of a chosen density.

Shell 1.9.0. The round-trip test carries gridCols and drops a value off the
scale.

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

README.md

@@ -273,7 +273,12 @@ below. Nothing is installed without someone clicking.
   place and shows nothing, then appears when it is; it is never replaced by
   the module's default tile. Saving over an existing name asks first, and
   the server keeps the copy it replaced: **Restore previous** in Admin →
-  Saved layouts swaps it back (and swapping again undoes that).
+  Saved layouts swaps it back (and swapping again undoes that). The menu's
+  **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.
 - Every tile has a **status dot**: green = live, amber = connecting,
   red = upstream unreachable (hover it for details). Tiles reconnect
   automatically.