Name an unnamed print stage "Preparing" on the card

New printers report stage numbers before Bambuddy learns their names,
and the H2C still has several. Those reached the printer card verbatim,
as "Unknown stage (72)" -- a number that means nothing to the person
reading it, on the one line that otherwise says what the printer is
doing.

Every stage that has turned out to be unnamed so far has been part of
the run-up to printing, so an unnamed one now reads as "Preparing".
That is the same literal stage 74 already carries rather than a second
spelling of the same idea, so a card cannot show two different words
for the same situation depending on which number the firmware picked.

Display only, and deliberately not pushed down into get_stage_name.
That function also feeds the stage-transition log line and the
once-per-session warning added to capture unnamed stages so they can be
named in a later release; there the number is the entire diagnostic
value, and replacing it with "Preparing" would hide the only thing that
reports these. Both paths are pinned by tests asserting they disagree
for an unnamed stage and agree for a named one, so a later tidy-up
cannot quietly collapse them.

The idle sentinels are untouched: 255 on A1/P1 and -1 on X1 mean "no
stage", not an unnamed one, and still resolve to nothing rather than
being swept up by the fallback.

Nothing keys logic off stg_cur_name -- it is display-only in the printer
card, the print dialog's printer selector and the stream overlay -- so
all three improve and none change behaviour. No i18n either way: the
whole stage table has always been English.

backend/app/services/printer_manager.py

@@ -8,7 +8,13 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from backend.app.models.printer import Printer
-from backend.app.services.bambu_mqtt import BambuMQTTClient, MQTTLogEntry, PrinterState, get_stage_name
+from backend.app.services.bambu_mqtt import (
+    STAGE_NAMES,
+    BambuMQTTClient,
+    MQTTLogEntry,
+    PrinterState,
+    get_stage_name,
+)
 from backend.app.utils.kprofile_lookup import build_slot_k_resolver
 
 logger = logging.getLogger(__name__)
@@ -1170,6 +1176,23 @@ def get_derived_status_name(state: PrinterState, model: str | None = None) -> st
     # X1 models use -1 for idle, A1/P1 models use 255 for idle
     # Valid stage numbers are 0-254
     if 0 <= state.stg_cur < 255:
+        # A stage number the table does not cover is named "Preparing" rather
+        # than "Unknown stage (72)". New models report stages before Bambuddy
+        # learns their names -- the H2C still has several -- and the card is
+        # the wrong place to say so: the number means nothing to the person
+        # reading it, and every stage that has ever turned out to be unnamed
+        # was part of the run-up to printing, so "Preparing" is both the more
+        # useful answer and the more likely one.
+        #
+        # This is display only, and deliberately not pushed down into
+        # `get_stage_name`. That function also feeds the stage-transition log
+        # line and the once-per-session warning that exists precisely to
+        # capture unnamed stages so they can be named later (bambu_mqtt.py
+        # ~4100) -- there the number is the entire diagnostic value, and
+        # replacing it with "Preparing" would hide the very thing that
+        # reports these.
+        if state.stg_cur not in STAGE_NAMES:
+            return "Preparing"
         return get_stage_name(state.stg_cur)
 
     # If not in RUNNING state, no derived status needed