Add missing MQTT stage names for H2D print preparation

H2D firmware reports stg_cur=74 during print preparation, which was
not in the STAGE_NAMES lookup table (sourced from BambuStudio, which
only goes up to 66). This caused "Unknown stage (74)" to display on
the printer card. Also adds stage 77 ("Preparing AMS") which exists
in BambuStudio source but was missing from the lookup.

CHANGELOG.md

@@ -5,6 +5,7 @@ All notable changes to Bambuddy will be documented in this file.
 ## [0.2.1b3] - Unreleased
 
 ### Fixed
+- **"Unknown stage (74)" on H2D During Print Preparation** — The H2D firmware reports `stg_cur=74` during print preparation, but this stage was not in the stage name lookup table (which went up to 66, sourced from BambuStudio). Now maps stage 74 to "Preparing". Also added stage 77 ("Preparing AMS") which was present in BambuStudio but missing from the lookup.
 - **Developer Mode Detection Always Reports Null** — The MQTT `fun` field is an integer in the JSON payload, but the parser used `int(value, 16)` which requires a string argument. This raised `TypeError` on every message, silently caught by the exception handler, so `developer_mode` was never set. Now handles both integer and hex string formats.
 - **File Manager Rename Doesn't Update Displayed Name** ([#460](https://github.com/maziggy/bambuddy/issues/460)) — Renaming a file in the File Manager updated the `filename` field but not `file_metadata.print_name`, which the UI uses as the primary display name. Since `print_name` is extracted from inside the 3MF at upload time, it always took precedence over the renamed `filename`. The rename endpoint now also updates `print_name` in the file metadata when present.
 - **Finish Photo Not Captured When Archive Has No Source 3MF** ([#484](https://github.com/maziggy/bambuddy/issues/484)) — When a print completed but the 3MF source file wasn't downloaded from the printer (e.g. FTP download failure), the archive's `file_path` was null. The finish photo capture silently skipped because it derived the save directory from `file_path`. Now falls back to `archive/{id}/` so the photo is captured regardless.

backend/app/services/bambu_mqtt.py

@@ -242,6 +242,8 @@ STAGE_NAMES = {
     64: "Preparing Hotend",
     65: "Calibrating nozzle clumping detection",
     66: "Purifying the chamber air",
+    74: "Preparing",  # Seen on H2D during print preparation
+    77: "Preparing AMS",
 }