feat: allow choosing another printer when printing files from printer storage

[jactan77]

Problem

When printing a file from a printer's storage (device page → files / print history → Print), the printer combobox in SelectMachineDialog is unconditionally disabled (FROM_SDCARD_VIEW mode). If the source printer is busy, the dialog shows "The printer is busy on other print job" and there is no way to print the job on another idle printer — the printer list cannot even be expanded.

Solution

• The printer list stays enabled in FROM_SDCARD_VIEW mode (it is only disabled when the source printer's file system is not available).
• Selecting the source printer keeps the existing behavior (start_sdcard_print, the printer prints its local file).
• Selecting a different printer triggers a transfer: Studio downloads the .gcode.3mf from the source printer via PrinterFileSystem (progress + cancel supported in the dialog's status bar, previously downloaded files are reused), then sends it to the selected printer through the regular upload path — new PrintJob type from_sdcard_transfer (LAN FTP with cloud fallback), preserving the plate index, AMS mapping and print options.
• The printer-model mismatch warning now also covers cross-printer sends.
• MediaFilePanel passes the source printer's file system, file index and dev id to the dialog via a new Plater::set_sdcard_print_source().

Testing status

• Builds cleanly on Linux (Ubuntu 24.04 docker flow, BuildLinux.sh -s); no new compiler warnings in the touched files.
• The same-printer path is unchanged by design; the cross-printer path has been verified against a protocol-level printer simulator (MQTT/FTPS), not yet against real hardware — feedback from hardware testing is welcome.