feat(ui): filter already-added chars from participant dropdown

Character dropdown in add-participant picker excluded characters already
in the encounter. Prevents dup-add at the source — user can only select
chars not yet participating.

src/App.js

@@ -1352,7 +1352,7 @@ function ParticipantManager({ encounter, encounterPath, campaignCharacters, camp
                   className="mt-1 w-full px-3 py-2 bg-stone-700 border-stone-600 rounded text-white"
                 >
                   <option value="">-- Select from Campaign --</option>
-                  {campaignCharacters.map(c => (
+                  {campaignCharacters.filter(c => !participants.some(p => p.type === 'character' && p.originalCharacterId === c.id)).map(c => (
                     <option key={c.id} value={c.id}>
                       {c.name} (HP: {c.defaultMaxHp || 'N/A'}, Mod: {formatInitMod(c.defaultInitMod)})
                     </option>