Character section rollup: clickable title, count, persisted collapse

Match campaigns rollup pattern. Title button toggles collapse (chevron
left, like campaigns). Character count in header. Collapse state persisted
to localStorage key ttrpg.charactersCollapsed.

TODO.md

@@ -50,19 +50,6 @@ lots of updates
 - `isActive = id => updatedParticipants.find(p => p.id === id && p.isActive)`
 - Returns participant obj (truthy) not boolean. Works by accident, fragile.
 
-### BUG: select campaign during active combat = screen flicker, no action
-- In active encounter, click different campaign → flicker, no nav, no end.
-- Either block (toast: end encounter first) or auto-end current + switch.
-- Decide UX before fix.
-
-
-### FEAT: generic/non-5e rules mode
-- Campaign/encounter ruleset toggle: `5e` vs `generic`.
-- Generic mode turns off death saves/state machine.
-- Generic mode allows negative HP.
-- 5e mode rejects negative damage/heal and clamps HP at 0.
-
-
 
 ### FEAT: clarify "Is NPC" in add-participant
 - Ambiguous label. May expand work based on what NPC means here (ally? monster?