Show a progress bar while the update command runs

[kyleinoregon]

Draft until #286 is merged: this branch is stacked on it, so the diff currently shows that commit as well. Only the second commit (src/update_progress.h, the Makefile default and the guarded #include in setting_panel.cpp) is new here.

Builds on #286.

What

When the update button is enabled (UPDATE_CMD), pressing it now spawns the update command detached with sp::Popen instead of blocking the UI thread, and shows a dialog with an lv_bar fed by a status file the command writes: a single <state>|<percent> line, with state one of downloading, flashing, rebooting, failed. The dialog polls the file every 500 ms from an lv_timer. The command's exit status stays authoritative: if it exits before reporting rebooting, the "Failed" dialog is shown, same as today.

The file path defaults to /run/cosmos-update.status (UPDATE_STATUS_FILE in the Makefile, right next to UPDATE_CMD; nothing needs to set it). Builds without UPDATE_CMD are untouched: the include and the header are guarded and compile to nothing.

Notes on the implementation:

• Header-only (src/update_progress.h), following the simple_dialog.h pattern in the tree.
• The dialog uses the multiline_message variant of create_configurable_dialog(). The default no-button dialog on a 272 px high screen is 95 px tall and the bar would be clipped; the taller variant leaves about 30 px spare.
• The command is spawned the same way call_command() runs it (split on whitespace, executed directly), just without waiting.

The producer side for COSMOS is OpenCentauri/cosmos#308.

Tested

• Elegoo Centauri Carbon (480x272, GUPPY_SMALL_SCREEN) on COSMOS 26.08.0 with the cosmos script change: the dialog appears immediately, percentage and bar track the real 124 MB download, "Installing update, do not power off!" during the flash, then "Your printer will restart shortly!" until the reboot.
• Failure path exercised with a stub script that reports progress and then exits 1 part way through: red "Update COSMOS Failed" dialog with OK.
• Built with UPDATE_CMD (COSMOS's variables) and without it (default configuration), both without warnings.

[pellcorp]

So Update is basically only for COSMOS now, the Simple AF project never used Update, and I have stopped supporting helper script. So really the question becomes, will the cosmos project actually use this? Are they going to change their update script to publish progress, cos if they are not, this seems pointless.

If they are committing to it, then I am all for this.

[pellcorp]

I think this should be guarded with UPDATE_BUTTON_STATUS_FILE too.

[pellcorp]

Let me be really blunt, if Cosmos commit to the update status file, then there is no point in making this optional, where the UPDATE button is enabled in the Makefile, its expected that an update will proceed this way with progress, no need to add more options.

If they are not going to do the update status file then I wont merge this, I will merge your other PR though as that is good for everyone :-)

[kyleinoregon]

Makes sense. I've reworked it so there's no separate switch: when UPDATE_CMD is set the update always goes through the progress dialog, and the status file path is just a Makefile default (UPDATE_STATUS_FILE, /run/cosmos-update.status, nothing needs to set it). The include is guarded now too. I also dropped the recipe change from the cosmos PR, so cosmos only needs the script in OpenCentauri/cosmos#308, which is the one that decides whether this is worth merging. Leaving this as a draft until they answer.

[kyleinoregon]

Closing in favour of #289: instead of parsing update-cosmos output on the screen, grumpyscreen now shows Moonraker's update_manager messages, following the cosmos discussion in OpenCentauri/cosmos#308.