[E-OWL-2] Roll history panel

[alxjrvs]

Part of #945

Phase 2: Roll history panel

Scope

A roll history panel inside the Owlbear extension showing the last N rolls for the current session. Persisted in localStorage — no Owlbear room state API, no backend. Stateless per session (resets on browser close).

Acceptance Criteria

• Roll history panel displays the last 20 rolls (configurable constant, not hardcoded)
• Each history entry shows: notation, total, and timestamp
• History persists across panel closes and reopens within the same browser session (localStorage)
• History resets on browser close (session storage is an acceptable alternative if localStorage is unavailable in the iframe sandbox)
• "Clear history" button empties the list
• Panel is scrollable and remains usable at 320px width with a long history list
• Depends on Phase 1

Implementation Notes

• Use localStorage keyed by a fixed extension ID — no user auth needed
• If Owlbear's iframe sandbox blocks localStorage, fall back to in-memory state (log the limitation in the README)
• Phase 2 shared roll log (visible to all players via OBR.room.setMetadata) is explicitly deferred — this phase is local-only history
• When Phase 3 shared log is eventually designed, evaluate OBR.room.setMetadata as the persistence layer (purpose-built for per-room state, no backend needed)

Out of Scope

• Shared roll log visible to other players in the room (Phase 3)
• Game system selector
• Owlbear room state API