fix(imex): code-review nits — null check, cache-key precision, lambda capture

Four quick-fix items surfaced by pre-PR self-review.

GCodeViewer.cpp:1607
  Null-check get_curr_plate() before dereferencing. Other call sites in
  the file already guard; this was the only unguarded one in the IMEX
  layer-preview path. In practice m_plate_list always has a plate, but
  the inconsistency is easy to fix and removes the only ungated deref.

PartPlate.cpp:build_imex_cache_key
  Cache key for IMEX zone geometry truncated nozzle_clearance_x/y to int
  before stringifying — a config change from 30.0 to 30.5 would not
  invalidate the cache. Match the *10 precision pattern already used for
  imex_carriage_margin so 0.1 mm steps invalidate correctly.

Plater.cpp:select_plate_by_hover_id (right-click popup)
  Two issues:
  1. Lambda captured `modes` by reference. PopupMenu() is synchronous
     today so the reference outlived the menu's event handling, but the
     pattern is fragile — anyone refactoring to async Popup() would
     silently dangle. Capture by value.
  2. Used wxID_HIGHEST + i for menu item IDs — standard wx anti-pattern
     because it can collide with other handlers listening in that range.
     Allocate per-item IDs via wxNewId() and look up the chosen mode by
     finding the event ID in a parallel vector. The lookup becomes O(N)
     instead of O(1) but N is small (mode count) and this is clicker
     latency, not a hot path.

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

src/slic3r/GUI/GCodeViewer.cpp

@@ -1605,7 +1605,7 @@ void GCodeViewer::render(int canvas_width, int canvas_height, int right_margin)
                         float bed_x_min, bed_x_max, bed_y_min, bed_y_max;
                         {
                             PartPlate* curr_plate = wxGetApp().plater()->get_partplate_list().get_curr_plate();
-                            const Pointfs& plate_shape = curr_plate->get_shape();
+                            const Pointfs& plate_shape = curr_plate ? curr_plate->get_shape() : Pointfs{};
                             if (!plate_shape.empty()) {
                                 bed_x_min = (float)plate_shape[0].x();
                                 bed_x_max = bed_x_min;