Add configurable Copy button to screenshot toasts

[hideoneric]

Summary

• add an opt-in Copy image action to screenshot preview toast overlays
• expose Copy in the Overlay Buttons shelf and layout editor, including accessible keyboard activation
• add an off-by-default Close preview after copying behavior setting
• keep the preview available after copy by showing temporary inline success or failure feedback; failure leaves Copy available to retry
• use a distinct outbound-arrow icon for the Office/Open with action so it is no longer confused with Copy

Validation

• dotnet restore OddSnap.sln
• dotnet list OddSnap.sln package --vulnerable --include-transitive
• dotnet format whitespace OddSnap.sln --verify-no-changes --no-restore
• dotnet format style OddSnap.sln --verify-no-changes --severity warn --no-restore
• dotnet format analyzers OddSnap.sln --verify-no-changes --severity warn --no-restore
• dotnet build OddSnap.sln -c Debug --no-restore -warnaserror
• dotnet test src/OddSnap.Tests/OddSnap.Tests.csproj -c Debug --no-build --logger "console;verbosity=minimal" (433 passed)
• bun run --bun --no-install lint:unused (site)
• bun run --bun --no-install build (site)

Scope note

The repository's current site lockfile has a pre-existing nanoid@3.3.16 audit finding. This PR intentionally does not change dependency or lock files; the feature work is limited to the toast Copy action and its settings.