Fix first-person camera target and paint HUD rebind after networked spawn

Point the first-person camera at the dedicated FirstPersonTarget anchor
(eye height) instead of the shared CameraTarget, in both the runtime
SetPlayer rebind and the editor ResolveReferences fallback.

Rebind the paint HUD to the locally spawned avatar's MeshPainter via a
new GameModeManager.PainterChanged event. Since the spawn-on-connect
refactor, no player exists at PaintHud.Start(), so the HUD bound to a
null painter (no brush masks shown, color/settings were no-ops).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Prefabs/Hider.prefab

@@ -16,10 +16,6 @@ PrefabInstance:
       propertyPath: GlobalObjectIdHash
       value: 708302045
       objectReference: {fileID: 0}
-    - target: {fileID: 4587432968399885263, guid: 97017ed3bb40aff4e885440d9a7336ee, type: 3}
-      propertyPath: m_Enabled
-      value: 1
-      objectReference: {fileID: 0}
     - target: {fileID: 5384024074013944205, guid: 97017ed3bb40aff4e885440d9a7336ee, type: 3}
       propertyPath: m_Name
       value: Hider