NSWorkspace.shared.open() silently drops #fragment anchors on file://
URLs, so "Fix It Myself" always landed on the Help Book index instead
of the relevant section. Replaced with GitSyncManualFixSheet, an
in-app sheet showing the real conflicting filenames and sync path.
Also indent conversation rows one level deeper than their containing
folder in the sidebar and conversation list, so nesting is visible on
the conversations themselves and not just the folder headers.
