Rework the parallel modes editor from the UI review

Addresses the interface notes on the IDEX/IQEX modes editor.

Add Mode moves from below the rows to the top of the panel, beside a "?" button
that now carries the overview text as its tooltip. At the bottom the button
shifted down the page every time a mode was added, so where it sat depended on
how many modes already existed. It is an Orca Button in the Confirm style, width
matched to the mode column it creates a row in, and the panel opens on the
legend rather than on a paragraph.

Remove moves out of the right-hand column, where it sat one icon away from
Edit -- a destructive control beside the one pressed most -- to under the name
field it deletes, and its icon becomes a boxed minus rather than an X, which
read as "close". Reset joins Edit in the right-hand column, which is now
top-aligned so the icons hold position regardless of row height. Tool tiles are
square at 24px, and the header spacer tracks that width so the column titles
stay over their columns when the grid changes shape.

The two text fields were landing on GTK's near-black default border, invisible
against the panel: measured 45,45,49 against a 43,43,43 background, where the
settings fields above use 74,74,81. wxTextCtrl cannot color its own border, so
each sits in a one pixel frame taking the color TextInput derives for the
theme, and carries wxBORDER_NONE so Windows and macOS do not draw a native edge
inside it. The G-code boxes also take the monospace face EditGCodeDialog uses.

Bed zone fills drop to roughly half opacity in the Standard theme. They cover
whole quadrants for a whole session, so at swatch saturation they dominate the
scene. The collision strip is dimmed less, since it marks where a head hits
something. The deuteranopia, tritanopia and high contrast themes keep their
alphas: those are chosen for discriminability, which is the opposite trade.

Also fixes the icon size never applying. All four ScalableButton call sites
passed eight arguments, so the size bound to use_default_disabled_bitmap and
bmp_px_cnt kept its default of 16. Both are passed now.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/PartPlate.cpp

@@ -1272,11 +1272,18 @@ void PartPlate::render_imex_zones(bool force_default_color)
     };
 
     // Standard (Okabe-Ito orange + sky blue)
+    // Alphas are deliberately low: these fills cover whole quadrants of the bed and sit under
+    // the model for the entire session, so at the saturation that reads well on a swatch they
+    // dominate the scene. The zone a tool owns only has to be legible, not loud.
+    // The three themes below keep their original alphas -- they are chosen for discriminability
+    // (and k_high_contrast explicitly for low vision), which is the opposite trade.
     static const IMEXTheme k_standard = {
-        { 0.337f, 0.706f, 0.914f, 0.45f },   // copy   — sky blue  #56B4E9
-        { 0.902f, 0.624f, 0.000f, 0.45f },   // mirror — orange    #E69F00
-        { 0.850f, 0.100f, 0.100f, 0.55f },   // danger — red
-        { 0.300f, 0.900f, 0.200f, 0.40f },   // margin — lime green
+        { 0.337f, 0.706f, 0.914f, 0.22f },   // copy   — sky blue  #56B4E9
+        { 0.902f, 0.624f, 0.000f, 0.22f },   // mirror — orange    #E69F00
+        // Dimmed less than the fills: this strip marks where a head collides, so it stays the
+        // most prominent thing on the bed even after the surrounding zones recede.
+        { 0.850f, 0.100f, 0.100f, 0.40f },   // danger — red
+        { 0.300f, 0.900f, 0.200f, 0.20f },   // margin — lime green
     };
     // Deuteranopia / Protanopia: avoids red-green confusion.
     // Danger strip uses strong blue-violet (red invisible to protanopes).