feat(chat): paste images into the composer

[bmc08gt]

Images pasted into the chat composer now stage as photo chips. Both long-press Paste and the iOS 27 "Paste from Photos" keyboard suggestion work, and they go through the same staging as images dropped on the bar (ChatPhotoStaging.stageProvided, renamed from stageDropped).

Why the field edits attributed text

The keyboard suggestion only offers an image to a text view with allowsEditingTextAttributes on. With it off, tapping the suggestion does nothing and no paste callback fires. It also bypasses the view controller's paste(itemProviders:), so the hook has to be the field's own pasteDelegate.

ComposerImagePasteHandler is that delegate. ChatScreenViewController installs it on the composer's field when editing begins, because SwiftUI can rebuild the field. Image items are taken out of the text and batched to onImagesPasted, and everything else pastes as usual.

The chat still sends plain text, so rich text stays out of sight:

• ChatScreenViewController.buildMenu(with:) removes Format from the edit menu.
• Pasted text drops its source font and color and takes the field's style.

Behaviour to know about

• In a chat that takes no photos, or while editing a message, a pasted image is dropped. Paste is still offered for an image-only clipboard there, matching how a drop is declined.
• On iOS 27 the suggestion's "Photo" title sometimes renders black in our field. It does the same in the Claude app but not in Notes. The field's text color and typing attributes are white, and neither pinning them nor keyboardAppearance = .dark changed it.