End and reset encounter

[gwillish]

Allow the GM to explicitly end an encounter and optionally reset it for reuse.

Scope

• "End Encounter" button in runner toolbar: navigates back to builder, session state preserved in SessionRegistry
• In the builder, when a session exists: show "Reset Session" option (confirmationDialog)
• "Reset Session" clears the session from SessionRegistry — next "Run Encounter" tap starts fresh
• Resetting does not modify the EncounterDefinition (roster, players, GM notes are unchanged)

Depends on

• Session auto-save and resume (Session auto-save and in-progress resume #39)

Acceptance criteria

• "End Encounter" returns to builder without destroying session
• "Reset Session" in builder clears the session with confirmation
• After reset, "Run Encounter" creates a new session from scratch
• Definition (adversary roster, players, notes) unchanged after reset

[gwillish]

Scoped design (based on decisions 2026-04-04)

Two distinct actions in the runner:

• Reset Session (already implemented as secondary toolbar action): discards current session state, creates a fresh session from the same definition, stays in the runner
• End Encounter: marks the encounter as done (or abandoned — no distinction needed). Clears the session from SessionRegistry and deletes the persisted session file from disk, then pops back to the encounter library. No summary or archive step required.

"End Encounter" should be a clearly labelled button, separate from Reset, in the runner toolbar or as a destructive confirmation dialog action.

Depends on

• Session auto-save and in-progress resume #39 (session persistence — End must delete the persisted file, not just clear in-memory state)