Fix #457: paste screenshots from the clipboard on macOS, Windows and the web

A screenshot on the clipboard didn't paste everywhere:
- macOS: the Paste menu item (which takes Cmd+V there) was enabled for text
  only, so with nothing copied in the app Cmd+V did nothing; and arboard
  reads the pasteboard's TIFF only. The pasteboard's types now enable Paste,
  and its PNG, else TIFF, pastes as it is.
- Windows: bitmaps came through CF_BITMAP and GetDIBits, which drops alpha.
  Paste now reads PNG, else CF_DIBV5, else CF_DIB, decoded with their colour
  masks and tables (a BMP file header in front, which image's headerless
  decoder gets wrong for masks after a BITMAPINFOHEADER), opaque when the
  alpha is zero throughout as screenshots leave it, capped at 32768 px a side.
- Web: only SVG text pasted. The page's paste event now hands the first
  picture or file that is art to the app (VectorcraftApp::paste_from_host),
  pasted with the chord held, like the desktop's paste keys.
- Linux reads image/png already; its Paste menu item still waits for text
  (no cheap way to ask arboard for a bitmap), the paste keys take one.

Bitmaps paste as embedded images at the view centre, as before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -304,8 +304,9 @@ the same); the last one runs the paste with `swatchConflict`. `ui.dialog.cancel`
 
 Paste placement: through the app, `edit.paste` and `edit.pasteWithoutFormatting` without `center`, `dx` or `dy`
 paste at the centre of the view. The Paste menu items are enabled while the system clipboard holds something to
-paste (SVG, PDF, text or a bitmap on the desktop; SVG on the web), even with nothing copied in the app (looked at up
-to four times a second; `ui.menu.list` shows it). The Layers panel menu (≡ on
+paste (SVG, PDF, text or a bitmap on Windows; text or a bitmap on macOS; text on Linux, whose paste keys still take a
+bitmap; SVG on the web, whose paste keys also take pictures and files), even with nothing copied in the app (looked
+at up to four times a second; `ui.menu.list` shows it). The Layers panel menu (≡ on
 the dock's tab strip while Layers shows) lists the layer commands (New Layer…, Duplicate and Delete Selection,
 Options for Selection…, clipping mask, isolation, Locate Object, Merge Selected, Flatten Artwork, Collect in New
 Layer, Release to Layers, Reverse Order, Template, Hide/Outline/Lock Others or Show/Preview/Unlock All Layers), Paste
@@ -346,7 +347,8 @@ System clipboard formats: through the desktop app, `edit.copy` and `edit.cut` pu
 lists to the system clipboard, and every Paste command first loads what another app copied (`clipboard.importSvg`,
 `importPdf`, `importText` or `importImage`, centred in the view); an import error answers the Paste with that error
 and pastes nothing. Cmd+V also pastes a bitmap or PDF with no text beside it (on the key's release, as egui sends no
-Paste event for it).
+Paste event for it). Screenshots paste on every platform: Windows reads `PNG`, else `CF_DIBV5`/`CF_DIB`, macOS the
+pasteboard's PNG or TIFF, Linux `image/png`, and the web the picture the browser's paste event carries (#457).
 
 File menu details: `file.openRecent1`…`file.openRecent30` open the recent files the `recentFilesCount` preference
 lists (0 hides them all; `file.recentFiles` returns the listed paths), `file.reveal` shows the document's file in the

docs/mcp.md

@@ -1479,8 +1479,11 @@ physical size) or `clipboard.importText {text}` (point text in the default type
 (default: the first artboard); then any `edit.paste*` command pastes it. The desktop app does this itself: Copy and
 Cut publish the formats to the system clipboard, and a Paste outside text editing first reads what another app
 copied (SVG, then PDF, then text, then a bitmap; SVG markup in text counts as SVG). Pasting what VectorCraft copied
-keeps the lossless internal clipboard. Windows carries every format both ways; macOS and Linux carry one format
-(the text, else the PNG) and paste text and bitmaps. On the web only SVG text goes through the system clipboard.
+keeps the lossless internal clipboard. Windows carries every format both ways and pastes a bitmap from PNG, else
+from the device-independent bitmap screenshots copy (`CF_DIBV5`, then `CF_DIB`; alpha kept, opaque when it is zero
+throughout, at most 32768 px a side); macOS and Linux carry one format (the text, else the PNG) and paste text and
+bitmaps (macOS the pasteboard's PNG, else TIFF, as screenshots copy them). On the web Copy publishes SVG text, and
+Paste takes SVG text and the pictures and files a paste carries (a screenshot pastes as an embedded image).
 
 The `pasteTextFormatting` preference (`keep` or `plain`): with `plain`, text the Type tool copied pastes into type
 without its formatting, taking the style at the caret.