Timeline details panel: small polish leftovers #169

Leftovers from a UX audit of the Timeline details panel — none blocking, collected so they don't get lost:

1. Copy buttons appear on non-id values. The panel's top block renders through the legacy PanelDetailRow (frontend/src/components/PanelContent.jsx), which puts a copy button on every value — Operation: replace, Branch Name: main, the whole Changes sentence. The newer table sections follow a copy-on-ids-only rule. Fix needs an opt-out prop on PanelDetailRow, which is shared with the Graph and Metadata pages, so it should be its own careful change.
2. Header timestamp is dev-grade. Aug 18, 2026, 10:55:27.443 +03:00 — milliseconds and UTC offset. It's the shared legacy format (Graph panel shows the same), so changing it should keep both panels consistent.
3. Unify unavailable-snapshot wording. Write rows say snapshot …2468 unavailable on the impact line while the details panel says (expired or not loaded) — one phrasing should win everywhere.