fix(vp): emit FINISH after FTP upload so Print-flow slicers unwedge (#1280)

Bambuddy's VP supports two slicer flows: Send (file upload only — what
  queue/immediate/review modes are designed for) and Print (file upload
  + start-print, intended for proxy mode). When a user clicks Print
  against a non-proxy mode the VP must still respond gracefully — the
  file is fine to receive, just the start-print never happens. Instead
  the slicer wedged at "Downloading...(0%)" and blocked the next
  dispatch with "The printer is busy with another print job".

  Cause: on_file_received transitioned gcode_state PREPARE -> IDLE
  directly. Print-flow slicers watch the state cycle and only release
  their in-flight-job lock on PREPARE -> ... -> FINISH (or FAILED).
  PREPARE -> IDLE looks like "printer abandoned my job" and keeps the
  prior job pinned in the slicer's memory.

  Fix: transition PREPARE -> FINISH with prepare_percent=100. The 1-Hz
  periodic status push broadcasts the new state to every connected
  slicer within a second. Send-flow slicers don't watch this state so
  the change is a no-op for them; Print-flow slicers see the FINISH
  they were waiting for and unwedge.

backend/app/services/virtual_printer/manager.py

@@ -217,9 +217,18 @@ class VirtualPrinterInstance:
         else:
             await self._queue_file(file_path, source_ip)
 
-        # Reset MQTT status back to IDLE
+        # Signal job completion to the slicer. Send-flow slicers don't watch the
+        # post-upload state and would be happy with anything; the Print flow
+        # (intended for proxy-mode VPs, but users sometimes click it against
+        # queue/immediate/review modes too — #1280) watches the gcode_state
+        # cycle and only releases its in-flight-job lock when it sees FINISH.
+        # Going PREPARE → IDLE wedges the slicer's UI at "Downloading...(0%)"
+        # and blocks the next dispatch with "busy with another print job".
+        # PREPARE → FINISH satisfies both flows. prepare_percent=100 also
+        # unfreezes the slicer's "Downloading X%" progress bar which it ticks
+        # against the same field during the upload window.
         if self._mqtt and file_path.suffix.lower() == ".3mf":
-            self._mqtt.set_gcode_state("IDLE")
+            self._mqtt.set_gcode_state("FINISH", filename=file_path.name, prepare_percent="100")
 
     async def on_print_command(self, filename: str, data: dict) -> None:
         """Handle print command from MQTT."""