Use IDEX/IQEX in the user-facing strings

Closes review comment 10.

`461c69c83e` settled this in April — IMEX internally, IDEX/IQEX as the user-facing label — but the
UI strings were never converted. Every translated string naming the feature now reads IDEX/IQEX:
41 occurrences across the printer and process option labels and tooltips, the modes editor, the
plate mode indicator, the pre-slice warnings, the placement refusals and the slicing errors. The
reviewer listed eight; the rest were in the same class.

Nothing else moves. The config keys keep the `imex_` spelling — `is_imex`, `imex_mode_names`,
`imex_parallel_mode` and the rest are on-disk format in existing printer presets and 3MF projects,
so renaming them would break every profile and project already saved. C++ identifiers, filenames,
comments and test names keep IMEX as well: it stays the internal name of the subsystem, which is
what covers the topology space (one gantry with 2-4 tools, 2x1 and 2x2 grids) that neither acronym
names on its own. Where a tooltip quotes a key, the key spelling is preserved and only the feature
word around it changed.

The `is_imex` tooltip is reworded rather than substituted: it already named the hardware families
parenthetically, so a literal replacement would have said IDEX/IQEX twice in one sentence.

No translation impact — no IMEX string had reached OrcaSlicer.pot or any catalogue, so there is
nothing to migrate. One test asserted on the old error text and now matches the new one.

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

src/libslic3r/Print.cpp

@@ -1377,7 +1377,7 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
                     StringObjectException err;
                     // "Mixed filament" is the term the rest of the UI uses -- the button that
                     // creates one, and the sibling refusal for the wipe tower filament.
-                    err.string = L("Mixed filaments are not supported in IMEX parallel modes. "
+                    err.string = L("Mixed filaments are not supported in IDEX/IQEX parallel modes. "
                                    "Switch this plate to Primary mode.");
                     err.object = m_objects.front();
                     return err;
@@ -1442,7 +1442,7 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
 
                 StringObjectException err;
                 err.string = Slic3r::format(
-                    L("IMEX mode \"%1%\" prints with T%2%, but this plate's filaments are on %3%. "
+                    L("IDEX/IQEX mode \"%1%\" prints with T%2%, but this plate's filaments are on %3%. "
                       "Assign a filament loaded on T%2%, or switch this plate to Primary mode."),
                     parallel_mode, routing.primary_phys, routed_list);
                 // Gives the notification a "Jump to <object>" link, which selects the object