pattern_grid: explicit side through PatternGrid::size

The grid's preferred size was the theme's fluid constant (264 px at the reference surface) capped by the width the parent allocates, so a caller could only shrink it: a lock screen that wants the grid to span most of a phone's width had no way to ask for it, and a container's max_width did nothing past the default. PatternGrid gains a size field with a size( impl Into<Length> ) builder; preferred_size resolves it through Canvas::resolve_geom like the other geometry overrides and still caps it at max_width, and map_msg carries it across. Without the builder the fluid default applies as before.
docs/widgets.md describes the builder next to hide_stroke and error; CHANGELOG lists it under Unreleased.

docs/widgets.md

@@ -368,6 +368,8 @@ repeat, or taking in the node a straight stroke crosses is the app's
 rule, not the widget's. `on_release( msg )` fires when the finger lifts.
 `hide_stroke( true )` draws only the dots, against shoulder surfing;
 `error( true )` paints the current path in the palette's `danger` colour.
+`size( len )` sets the side of the square grid (any `Length`); without it the
+theme's fluid default applies, capped by the width the parent allocates.
 
 It sizes itself square to `min( max_width, 264 )` design pixels; wrap it
 in a `container( … ).max_width( … )` to pin it.