V1.8.0: the fallen are hidden, and allies can revive the dead

- A dead character, or a monster or NPC at 0 HP, is out of the fight: the
  server no longer sends it to players, target marks included, and it has
  no turn. Healing or reviving brings it straight back.
- The DM's screen leaves the fallen out too until "Show dead" is ticked,
  which lists them in a group of their own, marked hidden.
- Players see dead characters by name in a Fallen box. The player whose
  character is on turn in a battle gets "Try revive" there if that
  character is on their feet and knows a spell that revives, at a level
  they can cast. The dead come back at 1 HP.
- Spells have a "Revives" tick in the campaign's spell list. Revivify and
  Raise Dead have it, in the library and in existing campaigns.
- A dead character cannot be added to an encounter until revived.
- All packages at version 1.8.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/CHARACTERS.md

@@ -334,7 +334,7 @@ HP bar, on their encounter card, in the quick stats and in the party lists.
 |---|---|---|
 | Dying | Rolling death saves | Three successes, three failures, healing, or being stabilised |
 | Stable | At 0 HP, no longer rolling | Healing wakes them; damage makes them Dying again |
-| Dead | Three failures, or one blow too big to survive | Only the DM's **Revive at 1 HP** |
+| Dead | Three failures, or one blow too big to survive | The DM's **Revive at 1 HP**, or an ally's reviving spell in a battle |
 
 **Rolling.** While Dying, type what the d20 showed and click **Death save**.
 The app does not roll it.
@@ -359,7 +359,10 @@ The app does not roll it.
 - **Healing** from 0, by any amount, puts the character back up and clears
   the tallies and Stable.
 - **Dead** characters cannot be healed: Heal is refused with a message. The
-  DM clicks **Revive at 1 HP**, then heals as normal.
+  DM clicks **Revive at 1 HP**, then heals as normal. In a battle, an ally
+  with a reviving spell can do the same on their own turn, and until then the
+  dead are hidden from the players' encounter screen: see
+  [ENCOUNTERS.md](ENCOUNTERS.md), section 6.
 - The **DM** can click a pip to set either tally by hand, to fix a mistake.
 
 Dropping to 0 always starts from empty tallies. None of this changes any