Session notes scratchpad in runner

[gwillish]

Add a session-scoped notes scratchpad to the runner, separate from the GM prep notes in the builder.

Scope

• Accessible from a toolbar button in the runner (sheet or sidebar depending on platform)
• Free-form text — intended for mid-combat notes (e.g. "player used daily ability", "environment effect applied")
• Saved to session state, not to EncounterDefinition
• Cleared when session is reset

Depends on

• Phase 4 complete

Acceptance criteria

• Notes accessible during active encounter without leaving runner
• Notes persist across app backgrounding
• Notes cleared on session reset
• Notes separate from builder GM Notes (definition-scoped)