Copy the selected clip with Command-C

[alvst]

⌘C in the bar copies the selected clip to the pasteboard, the same path the context menu's Copy already uses. Every successful copy now shows a brief "Copied to Clipboard" toast, so a copy that doesn't paste anywhere still gives feedback.

What changed

• ⌘C copies the selected clip; the context menu's Copy action and ⌘C both route through copyItem.
• A "Copied to Clipboard" toast appears after every successful copy and fades on its own.
• A copied clip is promoted to the front of history.

Screenshots

Before:

After:

Notes for review

• The promote-to-front is deliberate, not a side effect. Copying a clip back out of history is a re-use of it, and the monitor correctly ignores Pesty's own pasteboard writes, so without this the clip you just reached for stays buried where it was. It's the same ordering rule a fresh copy from any other app already gets.
• Merge-order note: alvie/pr-16-paste-reliability adds a byte-identical ClipboardStore.promoteCopiedItem(_:at:). Git places them in different regions of the file, so it will not conflict — it will produce two identical methods and a redeclaration error. Whichever of the two merges second, delete the duplicate.
• Out of scope: no toast for pastes, and no setting to turn the toast off.

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This feature should be bundled into v2.

Part of #80.