fix(prompt-input): paste a spreadsheet selection as text, not an image

[HardMax71]

handlePaste now returns before collecting files when the clipboard has both text/plain and text/html, which is what a spreadsheet or document selection looks like, so the textarea's default paste inserts the text. Screenshots and copied images keep attaching: for "Copy Image", Chrome, Firefox and Safari put the image and HTML on the clipboard but no text/plain (Chromium, Firefox, WebKit). A file copied in Finder brings its name as text/plain but no HTML, so it attaches too. I updated the feature list and added a changeset.

Two new tests in __tests__/prompt-input.test.tsx build the clipboard with a real DataTransfer. "pastes the text of a spreadsheet selection instead of its image" (text, HTML table and a PNG: nothing attached, default not prevented) fails on main and passes here. "attaches a copied file whose name is also on the clipboard as text" passes on both. The existing paste test's mock gets a getData. The full @repo/elements suite passes (960 tests).

Closes #509

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