Snapshot UI: expose restore-mode selection when restoring/importing a snapshot

Follow-up to #1248. Restore modes (exact vs compatible) now exist in the backend, but the mode is chosen implicitly:

- in-history restores default to **exact**
- staged/imported envelope restores default to **compatible**

There is currently no way for the user to override the default in the UI. The restore/import confirm flow should let the user pick the mode (with the current defaults preselected and a short explanation of each), and show what "compatible" adapted after the fact (it already emits adaptation notices).

Related: #1219 (snapshot content viewer) — the same surface that shows a snapshot''s pip packages + custom nodes would be a natural place to host the mode picker. If an "additive" mode is added later, it should appear here too.

Start by tracing the restore/import confirmation flow and reviewing the backend restore modes described in #1248. Compare it with the snapshot content viewer discussed in #1219 and the existing adaptation notices. Done means the UI exposes exact and compatible choices with the current defaults selected, explains them, and shows what compatible mode adapted.

Written by the indexing model from the issue text.