fix(paste): attach clipboard image when paste content is empty

[joestump]

Problem

On macOS, copying an image (e.g. from Preview) and pressing CMD+V in the chat editor does nothing. Terminal emulators intercept CMD+V and deliver the clipboard's text via bracketed paste; when the clipboard holds an image instead of text, the resulting tea.PasteMsg is strictly empty, and handlePasteMsg fell through the text-paste path and silently inserted nothing. The image-only clipboard is never checked.

Fix

Two changes in internal/ui/model/ui.go:

1. Empty-paste fallback: when the pasted content is strictly empty, handlePasteMsg now delegates to pasteImageFromClipboard (the same path as the ctrl+v PasteImage binding), so image data on the native clipboard is attached as a pending attachment. The check is a strict == "" rather than TrimSpace, so genuine whitespace-only pastes still insert as text. Like the key-bound path, the fallback is gated to image-capable models.

2. Warn on image-incapable models: both paste-image entry points (the key binding and the new fallback) previously returned nil silently when the current model didn't support images — a paste vanished with zero feedback, which made the macOS bug undiagnosable from the UX. They now surface a warning naming the current model ("gpt-4o-mini doesn't support images").

Tests

New internal/ui/model/paste_image_test.go covers:

• empty paste → routes to the clipboard-image fallback (asserted via function-pointer identity, without touching the real system clipboard)
• empty paste on an image-incapable model → warning, not the fallback
• ctrl+v on an image-incapable model → warning
• whitespace-only paste → still inserts as text (not swallowed by the fallback)
• non-empty paste → unchanged text insertion path

Verification

• go test ./internal/ui/... — all pass
• go vet ./internal/ui/... — clean
• gofumpt -l internal/ui/ — clean
• golangci-lint run ./internal/ui/... — 0 issues
• This fix has been running in a downstream fork for a few days; the empty-paste routing tests pin the mechanism without depending on host clipboard state.