feat(tijdlijn): unboost button — retract a boost (Undo Announce) from posts
The 🔁 on a timeline post is now a toggle: not boosted -> boost (Announce + mark
locally); boosted -> unboost (highlighted) which sends Undo(Announce) so followers'
servers remove the reblog, and clears the local flag. sendInteraction handles
'unboost' (Undo wrapping Announce, matched on actor+object — no record of the
original Announce needed). All operator-driven: your click sends it, nothing
automatic touches the fediverse.
Co-Authored-By: Claude <noreply@…>
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