feat: let the user choose where built firmware is saved

[Raffi1202]

Problem

Firmware built with the Build job type (for example a WiFi build, where the
binary is uploaded through the device's web interface afterwards) is copied into
a temporary directory created with os.tmpdir() and that directory is opened in
the file explorer.

The operating system cleans temporary directories up, so the binary has to be
moved somewhere safe manually, out of a path like
C:\Users\<user>\AppData\Local\Temp\TX_ESP32_a7f3k2\, before it disappears.
There is currently no way to tell the configurator where builds should end up.

Change

The destination of a build is now configurable in Settings → Firmware Output
Folder:

• Ask me where to save after every build (default) – a folder selection
  dialog is shown after every successful build
• Always save into the same folder – builds are copied into a folder chosen
  once, without further interaction
• Temporary folder – previous behaviour

In the first two modes a folder named after the firmware is created inside the
selected location (D:\ELRS\MyTX-3.5.6\), so builds do not overwrite each
other, and every artefact of the build is copied into it. The file explorer is
opened at the new location, exactly as it was before.

Cancelling the dialog leaves the firmware in the temporary folder, so nothing is
lost if the dialog is dismissed by accident.

Notes

• Only the Build job type is affected; flashing is untouched, since
  firmwareBinPath is only returned for builds.
• The copying itself happens in the main process (new SAVE_BUILD_OUTPUT IPC
  handler), the renderer only passes the localized dialog strings.
• CHOOSE_FOLDER accepts an optional title/message/default path now, so the
  settings screen can reuse it. Existing callers are unaffected.
• The setting is persisted in local storage next to the other application
  options.
• Only en translations are added, the remaining locales are managed by
  Crowdin.

If preferring no behaviour change out of the box, the default mode can be
switched to FirmwareOutputMode.TemporaryFolder in AppStateProvider — happy
to change that if you would rather have this opt-in.

Testing

• yarn typecheck and yarn lint: no new errors
• Ran the configurator from source on Windows 11