[KnifeLemon]

What does this change?

Closes #645.

A new preset, Merge images, puts several pictures into one image, one under another or side by side. The front and back of an ID card or a driving licence become one JPG to upload, in one step instead of Merge PDF and then Long image.

It works like Merge videos: dropping two or more pictures on the slice opens the merge order window, where the pictures are sorted (by name, as people count, then by hand with the arrows or Alt+↑ / Alt+↓). For pictures the window has a layout card below the list:

• Top to bottom / Side by side: the first picture goes at the top, or on the left.
• Same width (Same height side by side), on by default: pictures of different sizes are scaled to the widest (tallest) one so the edges line up. Off: each keeps its own size, centred.
• Gap (px): 0 to 1000, a number box without spinner buttons, like the crop window's W × H.
• Background: a colour picker and a hex box (as on the Theme page), or Transparent. A format without transparency (JPG) gets the preset's usual background colour, white, where it would be see-through.

The button is named after the preset ("Merge images"; for the existing merges it now says "Merge videos" / "Merge audio" instead of always "Merge PDF"). The choices go into the merge and are remembered in the preset for next time; Cancel starts nothing and remembers nothing.

The result is one image named after the first picture, {name}_merged, as Merge videos names its file, in the first picture's format (photo_merged.jpg). A first picture Filee can only read (HEIC, RAW, SVG, ...) gives a PNG. The preset's other picture options (resize, quality, grayscale, size limit) apply to the merged image; rotation and EXIF orientation apply to each picture before it is placed.

How it is built

• ImageOptions gets JoinIntoOne, JoinDirection, JoinFit, JoinGap and JoinBackground; Preset.JoinsImages says a preset merges pictures. The two enums are new fields with LenientEnumJsonConverter, so a value from a newer Filee reads as the default. Filee 2.1.0 reads such a presets.json: it skips the fields it doesn't know (there the slice converts the pictures one by one).
• IImageJoiner (Core), implemented by the ImageMagick engine (Magick/ImageJoiner.cs). The layout is worked out from the pictures' sizes first, then each picture is read, scaled and drawn onto the merged image, so only one picture at a time and the result are in memory (a few small pictures are kept from the first read instead of being read twice). Pictures are turned by their EXIF orientation and put into sRGB, so photos with a Display P3 or CMYK profile match.
• JobQueue.RunMergedImagesAsync: pictures ImageMagick doesn't read as they are (ICNS, JPEG XR, SVG and other drawings) become a plain PNG in the work folder first, without the preset's options, which apply once to the merged image. Writing the one output is shared with Merge videos (WriteJoinedAsync), so naming, progress, cancelling and undoing a failed write are the same.
• Size limit: the same 200 megapixels as Long image (PdfiumConverter.MaxStackedPixels, about 800 MB while the image is put together). Long image scales a longer stack down; a merged picture is meant to be seen at its size, so a merge beyond that is refused with a translated message and its size, e.g. "The merged picture would be too large, merge fewer or smaller pictures (72000 × 4000 px)". Twelve 24 MP photos side by side are refused, eight fit. The format's own limits (JPG 65,500 px, WebP 16,383 px) scale the result as for every other picture.

Donut placement

Multi-file tools live on back faces, so Merge images goes on the back of the images donut, right before Merge PDF. That face was full (12 slices), so GIF gives its slot: a still picture saved as GIF (256 colours) is the least useful slice there. GIF stays in the library and on the video donut's front, and can be put back in Settings → Toolbar.

Settings schema 53

Main is at 50; #617 takes 51 and another PR 52, so this uses 53. It may need renumbering at merge time depending on what lands first. The step:

• adds the preset to existing libraries after the media tools, as in a new library;
• puts it on the images donut's back face before Merge PDF: in a free slot when there is one, else in place of GIF. A face the user laid out without GIF, or with Merge images already on either face, is left alone;
• puts the option back when a Filee before 53 saved the library since and dropped it (as schemas 31 and 45 do for their presets).

Type of change

• Bug fix
• New feature or improvement
• New format or engine
• Translation
• Documentation
• Build, CI or dependencies

How was it tested?

• dotnet format --verify-no-changes
• dotnet build Filee.slnx -c Release
• dotnet test: Core 664, App 1449, Setup 196, Cli 39 and Platform 86 pass; Engines 1837 pass and 2 failed while every project ran at once (HwpxReaderTests.A_hundred_pages_are_read_in_well_under_a_second, a timing test, and AppLocalRuntimeTests.ProcessRunner_reports_a_missing_DLL_instead_of_the_exit_code); both pass when run again alone and touch nothing here.
  • MergeImagesTests (Core): the preset and its place on the donut, the schema 50 → 53 step (full face, free slot, a face the user laid out, the option put back), the presets file with the new fields and a newer Filee's values, and the queue with a fake joiner (order, first picture's format, PNG fallback, a picture that becomes PNG first without the preset's options, refusals).
  • MergeImagesEngineTests (Engines, real ImageMagick on generated pictures): top to bottom at the same width with a white gap; side by side at their own sizes, centred on a transparent background; a greyscale picture next to a JPG photo with a navy gap; each picture turned before the merge; an SVG merged with a photo; the preset's resize on the merged image; the layout maths; the 200 MP limit; background colours.
  • MergeOrderTests (App, headless): from the donut, the window shows the layout, the choices (side by side, own sizes, gap 6, transparent) give a 156 × 80 PNG and are remembered in presets.json; Cancel starts nothing and remembers nothing. The layout fits at the window's smallest size in en, de, ar (mirrored), ko, ru, hi, ne and th, in light and dark.
• Tried it in the app: not tried on a desktop yet

Screenshots (UI changes)

Headless renders (MergeOrderTests.Merge_images_layout_fits_at_its_minimum_size, written to tests/Filee.App.Tests/bin/<config>/net10.0/screenshots/merge-images-*.png) at the smallest size the window opens with for pictures (440 × 540): en, de, ar, ko, ru, hi, ne and th, light and dark, each also side by side with a transparent background. Reviewed: every choice whole and inside the card, Arabic mirrored, the list keeps two rows in German and Russian where the layout takes two lines.

Checklist

• New UI strings are in all src/Filee.App/Assets/i18n/*.json files (12 keys, 17 languages)
• New third-party code or binaries are listed in THIRD-PARTY-NOTICES.md (none)
• The PR does one thing (unrelated changes go in a separate PR)

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