feat(plot): custom paper sizes with printable margins

AutoCAD lets a plotter configuration carry user-defined sheets; a drawing
then names them like any other media (`Roll_24_(609.60_x_1500.00_MM)`).
The plot dialog now defines such sheets inline, under the Size row:
name, width/height, unit, and the printable margins in that unit. Added
sheets are selected at once, offered for every device after the device's
own list, remembered in the plot config across sessions, and written to
the layout with their own margins (a printer that reports the same size
still wins with its hardware margins). A selected user-defined sheet can
be removed; the sheet itself stays selectable in the drawing that uses it.

`CustomPaper` / `Margins` live in the paper catalogue with serde support;
`PlotDevice::margins_mm` consults the user's definitions. The Plot
dialog's form helpers (button and field styles, dividers, `label :
control` rows) move to `ui::style::form` so the next dialogs share them
instead of copying.

Tests: draft validation and seeding, catalogue naming of user sheets,
device margins with user definitions, and dialog round-trips (add,
select, persist, write, replace, remove, refuse an invalid size).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/style/mod.rs

@@ -10,6 +10,7 @@ pub mod scale_manager;
 pub mod anno_object_scale;
 pub mod style_manager;
 pub mod common;
+pub mod form;
 
 #[cfg(test)]
 mod common_tests {