feat: apply a saved rule to existing mail, confirmed and undoable

The dialog's Apply reaches MainWindow, which counts the messages the rule
would actually move, confirms with that count, then writes through the
worker. The rule is written one tag at a time inside one undo macro,
because the worker's effective set is per message: a single +a -b write
would make the undo strip a tag from messages that already had it. Each
undo entry is filled from its own confirmation, so it reverts exactly
what moved. Refused while a sync holds the write lock, checked again
after the confirmation closes.

Item 212.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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