feat(imex): polish UI per PR feedback + add parallel-mode safeguards

Felix14-v2 PR review feedback (https://github.com/OrcaSlicer/OrcaSlicer/pull/13086#issuecomment-4323696312)
plus the slice-time validation work that follows from his bug list.

UI polish:
- Capitalize "Primary" in display (right-click mode menu, plate tooltip).
  Sentinel stays lowercase for wire compatibility.
- Pre-slice-warnings checkbox now uses Orca's ::CheckBox so it matches the
  green toggle style of the rest of the IDEX/IQEX configuration page.
- DPI-scaled the IMEXModesCtrl (modes editor) sub-panels, button grid,
  text wraps, gcode textarea, and the ghost-tooltip swatch (imgui.scaled).
  Legend swatches sized to body-text height for visual balance.
- Primary mode tool buttons in the modes editor are now disabled (read-
  only): cycling roles on the IMEX-off mode is a no-op and confusing.
- Modes editor sub-panels now explicitly inherit the app's window-default
  dark colour so chromeless ScalableButtons don't render with a visible
  light box around their icons on GTK dark themes.
- Per-mode-line reset arrows in the modes editor: each row gets a small
  reset bitmap that snaps that row's name+tools+gcode triplet back to the
  saved preset's value. matches_config() guard on the page-level reload
  prevents the textbox-being-typed-into from being destroyed mid-keystroke.
- New View menu item "Show IDEX/IQEX Toolhead" — toggles the per-carriage
  footprint boxes during G-code preview playback. Gated to Preview tab +
  IMEX printer; backed by app_config so it persists.

Coordinated config migrations:
- imex_tool_layout and imex_viz_theme migrate from coString to coEnum
  (ImexToolLayout / ImexVizTheme). Existing wire format preserved so
  saved presets and 3MFs deserialize unchanged. Side-benefit: both now
  pick up standard Field rendering and so finally show reset arrows.

Slice-time safeguards (the validation half):
- imex_suppresses_bare_toolchange(parallel_mode, count): suppresses the
  slicer's bare T<n> at print-start in any IMEX parallel mode (the user's
  imex_mode_gcode + machine_start_gcode owns tool activation there). Mid-
  print T<n> emits normally — Print::validate blocks the configurations
  where mid-print T<n> wouldn't make sense. Applied to both code paths
  inside GCode::set_extruder (the long multi-extruder path AND the
  single-extruder path that fires when multiple_extruders=false).
- imex_multicolor_block_reason(): hard-stop validator returning a user-
  facing reason string when the active IMEX configuration can't physically
  support multi-color. Catches IDEX (1 tool/gantry), 2-tool-active IQEX
  (no within-gantry swap topology), and any MMU/AFC lane sharing among
  used filaments. Wired into Print::validate as a slice blocker, and into
  PartPlate::has_imex_multimaterial_conflict so the plater badge agrees
  with the slice block (no more false positives where the badge warns but
  the slice goes through).
- New imex_mode_types config option (parallel array to imex_mode_names)
  and imex_mode_type_for() helper. Mode-type tag drives behaviour: zones,
  ghosts, and validation interpret modes differently per type. Initial
  types: "primary", "copy", "mirror", "split". Split modes are explicitly
  designed for paired-gantry IQEX multi-color and bypass the gantry-pair
  check in imex_multicolor_block_reason (MMU sharing still blocks them).

Test coverage:
- New unit tests cover imex_suppresses_bare_toolchange (4 cases),
  imex_multicolor_block_reason (8 cases including IDEX, IQEX 2-/4-tool-
  active, MMU sharing, missing primary, Split type), and imex_mode_type_for
  (3 cases including legacy fallback). Total: 186 IMEX assertions across
  71 test cases, all passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/libslic3r/GCode.cpp

@@ -7676,7 +7676,15 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
             m_pa_processor->resetPreviousPA(m_config.pressure_advance.get_at(new_filament_id));
         }
 
-        gcode += m_writer.toolchange(new_filament_id);
+        // Same suppression as the long multi-extruder path: at print-start in IMEX
+        // parallel modes the user's mode_gcode + machine_start_gcode is responsible
+        // for tool selection, so the slicer's bare T<n> would be a duplicate. Mid-
+        // print toolchanges (count > 1) emit normally — Print::validate() blocks
+        // multi-color in configurations where mid-print T<n> wouldn't make sense
+        // (see imex_multicolor_block_reason).
+        const std::string toolchange_command = m_writer.toolchange(new_filament_id);
+        if (!imex_suppresses_bare_toolchange(m_imex_parallel_mode, m_toolchange_count))
+            gcode += toolchange_command;
         return gcode;
     }
 
@@ -7913,7 +7921,13 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
     //BBS: don't add T[next extruder] if there is no T cmd on filament change
      //We inform the writer about what is happening, but we may not use the resulting gcode.
     std::string toolchange_command = m_writer.toolchange(new_filament_id);
-    if (!custom_gcode_changes_tool(toolchange_gcode_parsed, m_writer.toolchange_prefix(), new_filament_id))
+    // See imex_suppresses_bare_toolchange() — only suppress the print-start initial
+    // T<n> in parallel modes (the user's imex_mode_gcode + machine_start_gcode owns
+    // tool activation there). Mid-print T<n> emits normally; Print::validate blocks
+    // multi-color setups where it wouldn't make sense.
+    const bool suppress_imex_bare = imex_suppresses_bare_toolchange(m_imex_parallel_mode, m_toolchange_count);
+    if (!custom_gcode_changes_tool(toolchange_gcode_parsed, m_writer.toolchange_prefix(), new_filament_id)
+        && !suppress_imex_bare)
         gcode += toolchange_command;
     else {
         // user provided his own toolchange gcode, no need to do anything