Add headless regression tests for Prop Editor functionality

- Implement tests for PropFile, including preset conversion, file dictionary conversion, round-trip serialization, version rejection, default values for missing keys, flattening, sanitization, and save/load functionality.
- Create tests for PropEditor, covering window title formatting, mode button text, safe saving, and recent file list management.
- Add tests for PropTheme, validating fallback behavior, invalid input handling, and default values for primitives and UI tokens.
- Introduce tests for ThumbnailCache, ensuring proper key formatting, PNG handling, and behavior with missing files.

props/box.prop

@@ -0,0 +1,116 @@
+{
+	"material_preset": "none",
+	"metadata": {
+		"created_at": "2026-09-19T16:03:21",
+		"modified_at": "2026-09-19T16:03:21"
+	},
+	"prop_name": "",
+	"shapes": [
+		{
+			"closed": true,
+			"fill_color": "ffffffff",
+			"outline_color": "#40290F",
+			"outline_width": 2.0,
+			"points": [
+				{
+					"x": 1258.0,
+					"y": 499.732421875
+				},
+				{
+					"x": 1230.26721191406,
+					"y": 603.232421875
+				},
+				{
+					"x": 1154.5,
+					"y": 678.999694824219
+				},
+				{
+					"x": 1051.0,
+					"y": 706.732421875
+				},
+				{
+					"x": 947.5,
+					"y": 678.999694824219
+				},
+				{
+					"x": 871.732727050781,
+					"y": 603.232421875
+				},
+				{
+					"x": 844.0,
+					"y": 499.732421875
+				},
+				{
+					"x": 871.732727050781,
+					"y": 396.232421875
+				},
+				{
+					"x": 947.5,
+					"y": 371.465148925781
+				},
+				{
+					"x": 1044.0,
+					"y": 507.732421875
+				},
+				{
+					"x": 1152.5,
+					"y": 367.465148925781
+				},
+				{
+					"x": 1230.26721191406,
+					"y": 396.232421875
+				}
+			],
+			"radius": 182.826955159505,
+			"reshaped": true,
+			"shape_type": "circle",
+			"vertex_flags": [
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0,
+				0
+			]
+		},
+		{
+			"closed": true,
+			"fill_color": "#8C6133",
+			"outline_color": "#40290F",
+			"outline_width": 2.0,
+			"points": [
+				{
+					"x": 1146.23413085938,
+					"y": 564.821594238281
+				},
+				{
+					"x": 1194.23413085938,
+					"y": 564.821594238281
+				},
+				{
+					"x": 1194.23413085938,
+					"y": 612.821594238281
+				},
+				{
+					"x": 1146.23413085938,
+					"y": 612.821594238281
+				}
+			],
+			"reshaped": false,
+			"shape_type": "polygon",
+			"vertex_flags": [
+				0,
+				0,
+				0,
+				0
+			]
+		}
+	],
+	"version": "1.0"
+}