fix: restore mixed-filament confirmation when switching to Preview tab

[iesteem]

Description

Restores the secondary confirmation dialog for mixed high/low temperature filaments when switching from the Prepare tab to the Preview tab.

Background

When a plate mixes high-temperature and low-temperature filaments and "Allow high/low temperature filament mixing" is enabled in Preferences:

• Clicking Slice Plate or Slice All correctly shows a confirmation dialog before slicing starts.
• Switching from Prepare to Preview used to show the same confirmation (in 2.3.5), but a regression introduced by fix: unify filament temp mixing blocking with plate error gating #589 silently dropped it — the switch auto-started slicing with no prompt.

(In 2.3.5 the Preview switch was implemented by re-posting the "Slice All" event, which incidentally carried the confirmation. #589 removed that repost — correctly, because it sliced all plates when only the selected plate should be sliced — but the confirmation was lost along with it.)

Changes

• Restore the confirmation on Preview tab switch (EVT_GLVIEWTOOLBAR_PREVIEW handler in Plater.cpp): when the switch would auto-start slicing and the current plate has mixed high/low temperature filaments, show the same single-plate confirmation dialog as the Slice Plate button.
  • Confirm → slice only the currently selected plate (single plate, not all plates) and stay on the Preview page.
  • Cancel → stay on the Prepare page; no panel switch and no slicing (the MainFrame tab is switched back, mirroring the 2.3.5 behavior).
• New predicate Plater::preview_switch_triggers_slice() (Plater.cpp / Plater.hpp): mirrors the slice branch conditions of Plater::priv::set_current_panel's do_reslice (non-empty model, printable instances on the current plate, stale slice result, no export/slicing in progress, model inside the print volume), so the dialog only appears when slicing would actually start.
• Align the Slice All dialog text with the Slice Plate dialog: filament_temp_mixing_warning_text_slice_all() now includes the same risk-warning sentence ("Mixed printing may result in extruder clogging, nozzle damage, or layer adhesion issues.") that was previously missing from the slice-all variant.

Hard-blocked plates (mixing not allowed via Preferences) keep the existing behavior: notification sync + no slicing, no dialog.

Screenshots/Recordings/Graphs

N/A (behavioral fix; dialog layout unchanged — only the slice-all warning text gained the risk sentence).

Tests

• Incremental build of libslic3r_gui passes (MSVC).
• Manual, mixed filaments + mixing allowed in Preferences:
  • Slice Plate → confirmation shown once; Confirm slices.
  • Slice All → confirmation shown once with per-plate breakdown incl. risk sentence; Confirm slices.
  • Prepare → Preview switch → confirmation shown once; Confirm slices only the selected plate and lands on Preview.
  • Prepare → Preview switch → Cancel → stays on Prepare, no slicing starts.
  • Preview tab re-clicked while already on Preview → no unexpected dialog.
  • Preview thumbnail-triggered reslice → existing guards unaffected.
  • Mixing NOT allowed in Preferences → hard block (notification, no dialog), unchanged.