Task: Show history as a unified diff in ores.shell

The shell's history output gains a unified-diff view: consecutive versions rendered in the familiar +/- format (or via an option such as --diff), consuming the same fields/changes history response as the Qt dialogs — demonstrating that frontends need no comparison logic.

• Shell history shows (or can show) a unified diff between consecutive versions.
• No field-comparison logic in ores.shell.
• Shell recipe updated; verified against the running system.

1. Extend the shell history command with a unified-diff rendering of the changes rows (and fields context), defaulting per UX discussion — either the default view or behind a --diff flag.
2. Reuse ores.diff types; no comparison logic in the shell.
3. For the unified-diff rendering itself, prefer an established vcpkg library over hand-rolling the format (survey candidates: e.g. diff-match-patch, dtl — pick whichever is maintained and fits the std+rfl dependency posture). Hand-roll only if nothing suitable exists in the registry.
4. Update the shell recipes documenting the history command.
5. Depends on the pilot task for the extended response (currency first; per-entity availability follows the rollout).

• Library survey outcome (Marco's decision: drop the library): dtl is available in vcpkg (v1.21) and diff-match-patch candidates exist, but both are diff computation libraries. After the currency pilot the server already supplies the computed changes rows, and the acceptance forbids comparison logic in the shell — so the unified-diff view is pure formatting of server rows (context lines from fields, +/- pairs from changes). No new dependency.
• UX: a separate currencies history-diff <iso> command beside the table view, as the cli menu library favours commands over flags.