Convert Export PNG button into Export dropdown with PNG and clipboard

[mucow24]

Summary

• Turns the toolbar's bright-blue Export PNG button into an Export dropdown with two destinations: PNG (existing download) and To clipboard (writes `image/png` via `navigator.clipboard.write([new ClipboardItem(...)])`).
• Trigger keeps the bright-blue glowing styling — only label + caret on the trigger, the download-arrow icon moves into the PNG menu item; a new clipboard glyph labels the second item.
• Click-to-close on the trigger blurs it so a subsequent mouse-leave actually clears the export-bounds cue (browser focus would otherwise pin it lit).
• "Copied to clipboard" status bubble (reuses existing `useStatusToast`) confirms success; clipboard API errors surface via the existing app error banner.

Test plan

• `npm run preflight` passes (lint, typecheck, 663 unit tests, 130 e2e tests)
• New unit test for `copyCanvasToClipboard` (mocks `navigator.clipboard.write` + `ClipboardItem`)
• New Toolbar test for the click-then-mouse-away cue regression
• New e2e test asserts clipboard write contains an `image/png` ClipboardItem starting with the PNG magic bytes
• Existing FL-04 / FL-10 / e2e specs updated to drive the new dropdown
• Manual: PNG download works, clipboard copy works (verified via paste), focus-mode cue clears correctly after click-then-leave

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