fix(ribbon): align combo panel dropdowns to a shared baseline

Style selectors (Standard, layer, Color/Linetype/Lineweight) sat at
different heights across ribbon panels because each combo panel
vertically centered its content: panels with fewer icon rows
(Multiline Text, Table) centered their dropdown differently than
panels with two rows (Dimensions, Multileader).

- Pin all combo panels (Layers, Properties, style combos) to the top
  of the fixed 3-row panel with a shared COMBO_PANEL_PAD, so every
  primary dropdown shares one baseline regardless of icon-row count
- Extract the duplicated tool-row and panel-container code into
  tool_row() / combo_panel_col() helpers + named spacing constants
- Bundle the per-view render state into RenderCtx, shrinking
  render_large/render_large_dropdown from 12-14 positional args to
  (item, &ctx)
- print_to_printer: silence the Windows-only dead-code warning on
  PrintOptions::quality with the platform split documented (the field
  is read by the CUPS `lp -o print-quality` path)

src/io/print_to_printer.rs

@@ -45,7 +45,10 @@ pub struct PrintOptions {
     pub printer: Option<String>,
     /// Number of copies (treated as at least 1).
     pub copies: u32,
-    /// Print quality label selected in the plot dialog.
+    /// Print quality label selected in the plot dialog. Read only on the CUPS
+    /// path (`lp -o print-quality=…`); on Windows the driver's own quality
+    /// setting wins, so the field is legitimately unread there.
+    #[cfg_attr(target_os = "windows", allow(dead_code))]
     pub quality: Option<String>,
     /// Controls applied while building the intermediate PDF.
     pub render: crate::io::pdf_export::PdfPlotOptions,