feat: make Sync follow the account being looked at

Item 101. The sync run was already account-aware for pending edits and
consulted the account dropdown for nothing, so looking at one account and
pressing Sync collected every one of them.

pendingSyncChannels() now reads the dropdown as well as m_editedAccounts.
The two are a UNION rather than one replacing the other, which is the whole
safety property: looking at one account while having edited another is
ordinary, and a run that dropped the edited account's channel would strand
that write with nothing on screen to say so. All accounts is the empty key
and narrows nothing, so a full fetch stays what an unselected window asks
for. The existing fallback is untouched: an account whose section names no
channel still widens the run rather than being silently skipped.

syncStartedText() is the visibility half. A run opened with "Syncing..."
whether it covered one account or all of them; it now names what it covers.
The exit paths overwrite the label with their own wording immediately, so
the parameter is defaulted rather than threaded through them.

Four tests. The narrowing one asserts the channel list is EMPTY before the
gesture, since empty is what a full fetch looks like and a test starting
from a narrowed state could not tell the fix from a window that had never
widened. Mutation-checked twice: ignoring the selection fails three of
them, and replacing the pending set instead of unioning with it fails the
union test alone, which is the version that looks correct and loses a
write.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KphFXTc2QajxXsHWyvGJ4R

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