fix(word-addin): unstick chat-history pagination and stop cross-document flashes

WHY THIS MATTERS
loadMore talked to the fetch effect solely through setOffset(chats.length).
Offset pagination over a list ordered by updated_at shifts whenever a
chat is bumped to the top, so a whole fetched page can be deduplicated
away -- leaving chats.length equal to the current offset. The
offset-keyed effect then never re-fires, and the requestPending guard
plus the loadingMore spinner stay stuck forever: pagination is dead
until the pane reloads.

WHAT IS AN OFFSET-PAGINATION SHIFT
Page N is defined as "rows N*size..N*size+size at query time". If a row
moves ahead of the cursor between requests (updated_at reordering),
page N+1 re-serves rows you already have; dedupe correctly drops them,
but any signal derived from list length no longer moves.

HOW IT WORKS
A monotonic requestId now accompanies the offset: loadMore bumps both,
and the fetch effect keys on the requestId, so every loadMore performs
exactly one fetch even when the numeric offset is unchanged. Dedupe
semantics and the history-changed subscription are untouched.

Also fixed here: switching document or storage scope while offset != 0
used to early-return after setOffset(0) without clearing state, so the
previous document's chats stayed on screen until the new fetch
resolved. The list and hasMore now reset before that early return.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015U1iPxsADQ2yuksNmDuvvW