Resume in-flight session on app launch

[gwillish]

Summary

If an encounter session was in progress when the app was last killed, the app should make it immediately obvious on launch and offer a fast path back into the runner.

Requirements

• On startup, after SessionRegistry loads persisted sessions (Session auto-save and in-progress resume #39), check for any active (non-stale) in-flight sessions
• If one exists, surface it prominently at launch — either:
  • Auto-resume: navigate directly into EncounterRunnerView for the in-flight session (most aggressive; appropriate if there is exactly one in-flight session)
  • Resume prompt: show a banner or sheet over the encounter library offering "Resume: " as a primary action
• If multiple in-flight sessions exist (unlikely but possible), show a list to pick from rather than auto-resuming
• A dismissed resume prompt should not re-appear; the session remains persisted until explicitly ended (End and reset encounter #40)

Design questions to resolve at implementation time

• Auto-resume vs. prompt: decide based on how jarring auto-resume feels in practice. Starting with a prominent banner/sheet is safer and lets the GM choose.
• Placement: banner over encounter library, or a dedicated "In Progress" section at the top of the library list?

Depends on

• Session auto-save and in-progress resume #39 (session persistence — sessions must survive the kill to resume)
• End and reset encounter #40 (end encounter — the in-flight indicator should disappear once the session is ended)