feat(pane): move the copy confirmation into the message pane

The user's preference after seeing item 115 ship: a small transient in the
bottom right of the pane with a checkmark, rather than a status bar message at
the far end of the window. A copy happens in the pane, so the confirmation
belongs there.

Three properties are load-bearing and each has a mutation that fails. The toast
is a hand-placed CHILD rather than a layout item, because it floats over the
message instead of taking a strip away from it: nothing reflows when it appears
and the text just copied does not jump. That is why resizeEvent() is overridden,
since a hand-placed child does not follow its parent. It is autoFillBackground
and painted from the theme's ToolTipBase/ToolTipText, so it stays readable over
a rendered message and follows the desktop theme the way the document already
does. And its timer is restarted rather than started, so a second copy gets its
own full reading time instead of inheriting what is left of the first.

The resize test was wrong on its first draft and passed against the mutation it
exists to catch. It grew the pane, which moves the right and bottom edges away,
so a toast left at its old position still satisfied "inside the pane"; measured
green with the reposition deleted. It shrinks now, where a stale position lands
outside the new rect, which is also what the user would see.

No new strings: the four messages are unchanged, only where they appear.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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