Choosing a picture clip when the panel can only copy, or pressing its Copy, empties the clipboard and says "Copied"

## What happens

A picture clip's `text` is `""`: the picture lives in a file (`PasteboardWatcher.swift:111-115`). Three paths treat it as text anyway.

1. **Return while the panel can only copy.** This happens when Accessibility is not granted, nothing is focused, or Uttrflow is in front. `resolving` returns `.copyOnly(clip, obstacle)` for any clip (`Sources/UttrflowUX/PanelKeyboard.swift:181-182`). `PanelOutcome.effect` turns that into `.copyAndSay(clip.text, …)` (`Sources/UttrflowUX/PanelEffect.swift:37`), and `AppDelegate` writes `""` with `announcingPasteboard.setText` (`AppDelegate.swift:856-861`, `:1099-1103`). `SystemPasteboard.setText` clears the pasteboard first (`Sources/UttrflowInput/SystemInput.swift:34-39`). What the user had copied is gone, and the panel says "Copied — press ⌘V".
2. **The row's Copy action.** It is offered on picture rows (`PanelPresentation.swift:436`) and calls `putOnClipboard(clip.text, …)` (`AppDelegate.swift:956-960`): the same empty write, and the panel closes.
3. **⌘Return on a picture.** `resolvingPlain` has no picture case (`PanelKeyboard.swift:187-195`), so the effect is `closeAndInsert("")`. It also skips the missing-picture check that `resolving` does.

"Make a note" is also offered on picture rows (`PanelPresentation.swift:459-464`) and would promote the empty text.

## Reproduced headlessly

```text
picture clip, insertion .clipboardOnly(.accessibilityNotGranted), .return
-> copyAndSay("", "Copied — press ⌘V. Turn on Accessibility and Uttrflow can paste for you.")
picture clip, insertion .atCaret, .returnPlain
-> closeAndInsert("")
row actions -> ["Insert", "Copy", "Pin", "Name", "Move", "Make a note", "Delete"]
```

## Why it matters

A user who has not granted Accessibility, which is the first-run state, picks a screenshot. They lose whatever was on their clipboard and are told the picture was copied. Pressing ⌘V pastes nothing.

## Acceptance criteria

- In copy-only mode, a picture clip puts its PNG on the clipboard through the announcing pasteboard's `setImage`. A missing file gives the existing "no longer on this Mac" notice.
- Copy on a picture row copies the picture.
- ⌘Return on a picture behaves like Return, including the missing-picture check.
- "Make a note" is not offered on picture rows.
- Tests in `Tests/UttrflowUXTests` cover all four, and a test asserts that no panel effect for a picture clip carries an empty string to the clipboard.

Start with the paths cited in Sources/UttrflowUX/PanelKeyboard.swift, PanelEffect.swift, PanelPresentation.swift, AppDelegate.swift, SystemInput.swift, and PasteboardWatcher.swift, then run the existing Tests/UttrflowUXTests suite. Trace picture clips through Return, ⌘Return, row Copy, and Make a note, including the existing missing-file behavior. Done means all four acceptance cases work and tests show no picture effect sends an empty clipboard string.

Written by the indexing model from the issue text.