feat(doc-scan): Query for FairScan, if built-in scanning not available

[PhilLab]

• Tests written, or not not needed

Built-in scanning is not available for all build variants - e.g. not for "generic", which is the flavor for the F-Droid release. It doesn't allow the non-reproducible TinyOpenCV build.

If this is the case, we are checking whether the open source scanning app FairScan is available (https://github.com/pynicolas/FairScan) and open that one for scanning.

This approach was selected based on the discussion here: #12624 (comment)

Result

Screen_recording_20260201_143916.webm

1. I thought about giving this menu entry a dedicated title, e.g. "Scan document from FairScan", but I decided against it - it's just more code to maintain and at the same time doesn't provide value to the user. Also it is not scalable: what if we add fallbacks to other scanner apps as well? So let's stick to the standard menu entry text.
2. I also had in mind to make this menu entry an "Install FairScan" button, in case there is no appscan and FairScan also is not installed. But that felt too intrusive.

Behind the scenes

1. To simplify the detection of the appscan project, a reflection-based approach was implemented. See the messages of the first two commits for why I am proposing this. If you don't agree with this approach, I can remove the commits from the PR, they don't impact the feature itself
2. FairScan delivers a short-lived URI which we then use to upload the file. Originally, the filename is an "ugly" unix timestamp - you can also see this in above recording, from a prior attempt: 1769952395766.pdf.
   • To make this a little prettier, the filename is transformed to our standard timestamped name, just like we do for images and videos captured via the camera intent.
   • Technically, this is done with the same approach of a fileDisplayNameTransformer as in my other current PR When uploading a single file, offer input field to change file name #16298.
3. Notably, FairScan's Intent feature is still experimental (see their README) but I'd still propose to include it, because:
   • For Non-F-Droid users, all stays the same and uses the built-in scanner
   • For F-Droid users, the menu entry will only appear if FairScan is installed, so the area of impact is quite small.
   • The intent integration is very basic so it is not very likely that FairScan's next iteration will change it in a way which is incompatible with this.
   • If it breaks by changing the intent URI - the button will simply not show anymore so we are not worse off than the status-quo.
   • If it breaks otherwise, then we are still not worse off than having no scanner in the F-Droid variant at all.

[github-actions]

blue-Light-Screenshot test failed, but no output was generated. Maybe a preliminary stage failed.

[github-actions]

APK file: https://www.kaminsky.me/nc-dev/android-artifacts/16427.apk



To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.