Web UI: config editor — baseline diff view + session snapshot history #154

Idea

Two editor features that build directly on the #116 staleness machinery:

1. Baseline diff view. The editor already loads the baseline text and hashes it for the staleness guard (ConfigEditorModal.tsx:359-364), but only the hash is kept. Keep the baseline text in a ref next to baselineHashRef and add a "Diff" toggle that renders a line-level diff (small pure LCS util in utils/) between baseline and draft before Apply/Save — users see exactly what will be written, complementing the Problems panel from #105/#111.

2. Session snapshot history. The editor tracks a save baseline per save (markSaved, lines 723-766). Add a "History" entry to the header listing the session's saved snapshots (text + timestamp kept in a ref stack); restoring one reuses the existing Revert/pendingAction confirm flow and the swap-doc path (which #147 will have made undo-friendly). Purely client-side — no server change needed for session history. (Disk-side history would be a separate, larger feature.)