fix(poc): resolve evaluator and review findings

- Add RectangleShape2D to Character and InteractiveObject collision areas
- Fix HUD button signal connections in _ready()
- Fix character_placed signal emitting global_position
- Extract DEFAULT_DRAG_RADIUS constant in DragDropComponent
- Type Variant on JSON parsed variable in SaveManager
- Extract music symbol constants in HUD
- Refactor duplicated drag input code in InputManager

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scenes/characters/Character.tscn

@@ -1,8 +1,11 @@
-[gd_scene load_steps=3 format=3 uid="uid://cozypaw_char"]
+[gd_scene load_steps=4 format=3 uid="uid://cozypaw_char"]
 
 [ext_resource type="Script" path="res://scripts/characters/character.gd" id="1_char"]
 [ext_resource type="Script" path="res://scripts/systems/drag_drop_component.gd" id="2_drag"]
 
+[sub_resource type="RectangleShape2D" id="RectangleShape2D_char"]
+size = Vector2(64, 80)
+
 [node name="Character" type="Node2D"]
 script = ExtResource("1_char")
 character_id = "bunny_01"
@@ -44,3 +47,5 @@ script = ExtResource("2_drag")
 input_pickable = true
 
 [node name="CollisionShape" type="CollisionShape2D" parent="CollisionArea"]
+shape = SubResource("RectangleShape2D_char")
+position = Vector2(0, -40)