Support non-0.4mm nozzles in AMS Slot config + guard dispatch (#1899)

The Configure AMS Slot picker was hardwired to 0.4mm (nozzleDiameter
prop never passed from PrintersPage / SpoolBuddyAmsPage), so a 0.6
machine could only set 0.4 profiles on its trays. Resolve the real
installed nozzle per-AMS (ams_extruder_map on dual-nozzle) and pass it
in. Separately, nothing validated the sliced nozzle against the
installed one, so a mismatch reached the printer as a cryptic HMS
_8012 "Failed to get AMS mapping table". Add a fail-safe pre-dispatch
guard in _start_print that fails the item with an actionable message
before upload; no slice diameter or no reported nozzles = no-op.

backend/app/services/print_scheduler.py

@@ -144,6 +144,50 @@ def _canonical_filament_type(ftype: str) -> str:
     return _FILAMENT_EQUIV_MAP.get(upper, upper)
 
 
+def _installed_nozzle_diameters(status) -> list[float]:
+    """Parse the installed nozzle diameters from a PrinterState (#1899).
+
+    Returns the diameters the printer actually reports (e.g. [0.4] single-nozzle,
+    [0.4, 0.6] dual-nozzle), skipping the empty-string defaults that populate a
+    NozzleInfo before MQTT fills it in. An empty list means "the printer hasn't
+    told us its nozzle hardware" — callers must treat that as unknown, not as a
+    mismatch, so we never block a print on missing data.
+    """
+    diameters: list[float] = []
+    for nozzle in getattr(status, "nozzles", None) or []:
+        raw = getattr(nozzle, "nozzle_diameter", "") or ""
+        try:
+            value = float(raw)
+        except (TypeError, ValueError):
+            continue
+        if value > 0:
+            diameters.append(value)
+    return diameters
+
+
+def _nozzle_mismatch_message(sliced_nozzle: float | None, installed: list[float]) -> str | None:
+    """Return an actionable error message when the sliced nozzle can't be
+    printed on any installed nozzle, else None (#1899).
+
+    Fail-safe: returns None whenever we lack the data to judge — no sliced
+    diameter, or the printer reported no nozzles — so a print is only ever
+    blocked on a POSITIVE mismatch. On dual-nozzle printers a match against
+    EITHER installed nozzle passes (a 0.6 slice is fine if one hotend is 0.6).
+    The 0.05 tolerance absorbs float noise while staying well inside the 0.2
+    gap between adjacent nozzle sizes (0.2/0.4/0.6/0.8).
+    """
+    if not sliced_nozzle or not installed:
+        return None
+    if any(abs(d - sliced_nozzle) < 0.05 for d in installed):
+        return None
+    installed_str = " / ".join(f"{d:g}mm" for d in installed)
+    return (
+        f"File sliced for a {sliced_nozzle:g}mm nozzle, but the printer has "
+        f"{installed_str} installed. Re-slice for the installed nozzle, or "
+        f"install the matching nozzle before printing."
+    )
+
+
 class PrintScheduler:
     """Background scheduler that processes the print queue."""
 
@@ -2629,6 +2673,47 @@ class PrintScheduler:
             await self._power_off_if_needed(db, item)
             return
 
+        # Nozzle-diameter mismatch guard (#1899). A file sliced for one nozzle
+        # size dispatched to a printer with a different nozzle installed is
+        # rejected by the firmware with a cryptic HMS ("Failed to get AMS mapping
+        # table" 0700_8012, or "nozzle diameter … not consistent" 0500_4038) that
+        # gives the user no idea what went wrong. Catch it here, before we spend
+        # time preheating and uploading, and fail with an actionable message.
+        # Fail-safe by construction: only a POSITIVE mismatch blocks — when the
+        # slice carries no nozzle diameter (archive.nozzle_diameter is None) or
+        # the printer hasn't reported its nozzles yet, we fall through and let the
+        # print proceed exactly as before. On dual-nozzle printers (H2D) a match
+        # against EITHER installed nozzle passes, so a 0.6 slice is fine as long
+        # as one of the two hotends is a 0.6.
+        sliced_nozzle = archive.nozzle_diameter if archive else None
+        if sliced_nozzle:
+            installed = _installed_nozzle_diameters(printer_manager.get_status(item.printer_id))
+            mismatch_msg = _nozzle_mismatch_message(sliced_nozzle, installed)
+            if mismatch_msg:
+                item.status = "failed"
+                item.error_message = mismatch_msg
+                item.completed_at = datetime.now(timezone.utc)
+                await db.commit()
+                logger.warning("Queue item %s: nozzle mismatch — %s", item.id, mismatch_msg)
+                await notification_service.on_queue_job_failed(
+                    job_name=filename.replace(".gcode.3mf", "").replace(".3mf", ""),
+                    printer_id=printer.id,
+                    printer_name=printer.name,
+                    reason=mismatch_msg,
+                    db=db,
+                )
+                try:
+                    await ws_manager.send_queue_item_failed(
+                        user_id=item.created_by_id,
+                        queue_item_id=item.id,
+                        printer_id=item.printer_id,
+                        reason="nozzle_mismatch",
+                    )
+                except Exception:
+                    pass
+                await self._power_off_if_needed(db, item)
+                return
+
         # Preheat / heat-soak (#1468) — fires before upload so the printer's
         # bed (and chamber, if applicable) is at temperature when the firmware
         # starts the actual print routine. Best-effort: any failure logs and