Add AC field (character, monster/npc, participant edit); layout polish

AC (Armor Class) optional field across all participant entry points:
- shared: ac field on makeParticipant + buildMonsterParticipant +
  buildCharacterParticipant, defaults null
- CharacterManager: defaultAc state + add form field + inline edit field +
  display in character list
- Monster add form: AC field
- EditParticipantModal: AC field next to Initiative
- ParticipantManager (DM list): AC badge on name row (sky-blue, stylized,
  large value, small label) for at-a-glance reading
- Player display: no AC (DM only)

Layout polish:
- Add participants form: 12-col grid, 5 fields single row (Init Mod, Initiative,
  AC, Max HP, HP Formula), shrunk from oversized fields
- Character add form: 12-col grid, name grows (col-span-6), Init Mod/AC/HP
  small right-aligned, order matches add participants
- Character inline edit: labels added (Name/HP/Init Mod/AC), name flex-grows
- HP Formula: label trimmed (example moved to placeholder 'e.g. 2d6+9'),
  Reroll button in edit modal
- ParticipantManager init input shrunk (w-8, centered)

Tests: 6 new AC builder tests (turn.ac.test.js). Existing test labels updated
for renamed fields.

shared/tests/turn.ac.test.js

@@ -0,0 +1,35 @@
+// AC field on participant + builders.
+const shared = require('@ttrpg/shared');
+const { makeParticipant, buildMonsterParticipant, buildCharacterParticipant, rollHpFormula } = shared;
+
+describe('AC field', () => {
+  test('makeParticipant accepts ac', () => {
+    const p = makeParticipant({ name: 'Orc', type: 'monster', initiative: 10, maxHp: 15, currentHp: 15, ac: 13 });
+    expect(p.ac).toBe(13);
+  });
+
+  test('makeParticipant ac defaults null', () => {
+    const p = makeParticipant({ name: 'Orc', type: 'monster', initiative: 10, maxHp: 15, currentHp: 15 });
+    expect(p.ac).toBeNull();
+  });
+
+  test('buildMonsterParticipant accepts ac', () => {
+    const { participant } = buildMonsterParticipant({ name: 'Goblin', maxHp: 7, initMod: 2, ac: 15 });
+    expect(participant.ac).toBe(15);
+  });
+
+  test('buildMonsterParticipant ac defaults null', () => {
+    const { participant } = buildMonsterParticipant({ name: 'Goblin', maxHp: 7 });
+    expect(participant.ac).toBeNull();
+  });
+
+  test('buildCharacterParticipant accepts character ac', () => {
+    const { participant } = buildCharacterParticipant({ id: 'c1', name: 'Fighter', defaultMaxHp: 30, defaultInitMod: 2, defaultAc: 18 });
+    expect(participant.ac).toBe(18);
+  });
+
+  test('buildCharacterParticipant ac defaults null when unset', () => {
+    const { participant } = buildCharacterParticipant({ id: 'c1', name: 'Fighter', defaultMaxHp: 30 });
+    expect(participant.ac).toBeNull();
+  });
+});

shared/turn.js

@@ -361,6 +361,7 @@ function makeParticipant(opts) {
     deathSaveSuccesses: opts.deathSaveSuccesses || 0,
     deathSaveFailures: opts.deathSaveFailures || 0,
     hpFormula: opts.hpFormula || null,
+    ac: opts.ac !== undefined ? opts.ac : null,
   };
 }
 
@@ -377,12 +378,13 @@ function buildCharacterParticipant(character) {
       initiative: finalInitiative,
       maxHp,
       currentHp: maxHp,
+      ac: character.defaultAc !== undefined ? character.defaultAc : null,
     }),
     roll: { roll: initiativeRoll, mod: modifier, total: finalInitiative },
   };
 }
 
-function buildMonsterParticipant({ name, maxHp, initMod, asNpc, hpFormula }) {
+function buildMonsterParticipant({ name, maxHp, initMod, asNpc, hpFormula, ac }) {
   const initiativeRoll = rollD20();
   const modifier = initMod !== undefined ? initMod : MONSTER_DEFAULT_INIT_MOD;
   const finalInitiative = initiativeRoll + modifier;
@@ -406,6 +408,7 @@ function buildMonsterParticipant({ name, maxHp, initMod, asNpc, hpFormula }) {
       maxHp: hp,
       currentHp: hp,
       hpFormula: hpFormula || null,
+      ac: ac !== undefined ? ac : null,
     }),
     roll: { roll: initiativeRoll, mod: modifier, total: finalInitiative, hpRoll },
   };