fix: widen undo dialog and polish its summary line

The undo/redo confirmation dialog capped at 60 columns and wrapped
even a short insert on wide terminals, showed a lowercase "snapshot
taken", a raw ISO-8601 timestamp, and lowercase yes/no labels.

Grow the dialog to fit its longest line (bounded 34–100), capitalise
Snapshot/Yes/No, and render the snapshot timestamp in local time,
human-formatted (24 May 2026, 11:00) via a new chrono dependency
(clock feature only; English month names). Yes/No capitalisation also
applies to the rebuild-confirm dialog.

Cargo.toml

@@ -11,6 +11,10 @@ publish = false
 [dependencies]
 anyhow = "1.0.102"
 base64 = "0.22.1"
+# `clock` brings local-timezone support (UTC → machine local) for
+# the undo-dialog snapshot timestamp (issue #13). No locale feature:
+# month names stay English; `unstable-locales` is deliberately avoided.
+chrono = { version = "0.4.44", default-features = false, features = ["clock"] }
 crossterm = { version = "0.29.0", features = ["event-stream"] }
 csv = "1.4.0"
 directories = "6.0.0"