feat(gui): auto-save the open project every 5 minutes

[BenJule]

Summary

Auto-saves the open project every 5 minutes: a timer silently calls save when the project is dirty and already has a file path, gated behind the autosave_enabled config key.

(Reduced from the original three-feature PR as requested — see split note below.)

Note: there is no Preferences toggle for autosave_enabled yet, so it is off unless the key is set. Happy to add a UI toggle if you'd like.

[BenJule]

Apologies — this PR was closed by mistake on 2026-06-07, and that was an error on my side, not a deliberate decision to withdraw the change.

It happened as an unintended side effect of a branch cleanup in my fork: deleting the head branch automatically closed this PR. The contribution still stands. I have restored the branch and reopened the PR. Sorry for the noise and any confusion this caused.

[BenJule]

Force-pushed: rebased onto current master and reduced this PR to the feature's own source changes. The previous revision had inadvertently carried translation-file (.po/.pot) edits, generated web resources, and unrelated source changes (string fixes belonging to #10811/#10813/#10814) over from the branch base — those are now removed. Heads-up: this feature series shares a few central GUI files (e.g. GUI_App.cpp, MainFrame.cpp, Preferences.hpp), so there is some overlap between my feature PRs.

[tonghao-bbl]

Hi @BenJule These are three different features, can you split them into three PR?

[BenJule]

Thanks @tonghao-bbl — split done. This PR is now auto-save only; the other two features are their own PRs:

• Fine arrow-key steps (Alt 0.1 mm / Alt+Shift 0.01 mm) → feat(gui): fine object-move steps (Alt 0.1 mm / Alt+Shift 0.01 mm) #11246
• Dark-mode contrast fix (layer-range editor) → fix(gui): correct layer-range editor text contrast in dark mode #11247

While splitting I also removed code that had ridden along on this branch from the shared base and actually belongs to other PRs — hex colour input (#10816), dark-mode follow-system (#11241), the cloud-warning dialog (#10839), the title * indicator (#10837) and the MakerWorld full-site option (the now-closed #10838). Each of the three PRs is now a single focused commit on top of master.

[BenJule]

Thanks @tonghao-bbl - done, this is split now. This PR is narrowed down to just the auto-save feature (the two MainFrame.cpp/.hpp changes). The other two features that used to be bundled here are their own PRs:

• fine object-move steps (the arrow-key change): feat(gui): fine object-move steps (Alt 0.1 mm / Alt+Shift 0.01 mm) #11246
• dark mode theme selector: feat(gui): add dark mode theme selector (Light / Dark / Follow system) #11241

So each feature can be reviewed independently now.