Fix respawned corpse movement, faction hostility, and add WoW-canonical mob level colors

Reset NPC animation to idle when health goes from 0 to >0 (respawn), prevent
dead NPCs from being moved by server movement packets. Fix faction hostility
to check factionGroup Monster bit and individual enemy arrays, not just
enemyGroup. Add level-based mob coloring: grey (no XP), green (easy), yellow
(even), orange (hard), red (very hard) for target frame and selection circle.

include/rendering/character_renderer.hpp

@@ -121,6 +121,7 @@ private:
         int currentSequenceIndex = -1;  // Index into M2Model::sequences
         float animationTime = 0.0f;
         bool animationLoop = true;
+        bool isDead = false;  // Prevents movement while in death state
         std::vector<glm::mat4> boneMatrices;  // Current bone transforms
 
         // Geoset visibility — which submesh IDs to render