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.

prop_theme.json

@@ -0,0 +1,47 @@
+{
+  "version": "1.0",
+  "outline_widths": { "default": 2.0, "min": 0.5, "max": 32.0 },
+  "material_tints": {
+    "wood":      { "fill": "#8C6133", "outline": "#40290F" },
+    "rubber":    { "fill": "#E63333", "outline": "#590D0D" },
+    "cardboard": { "fill": "#D9B873", "outline": "#66521D" },
+    "metal":     { "fill": "#808C99", "outline": "#262E38" },
+    "none":      { "fill": "#8C6133", "outline": "#26201E" }
+  },
+  "primitive_defaults": {
+    "box":      { "size_x": 48.0, "size_y": 48.0 },
+    "ball":     { "radius": 24.0 },
+    "plank":    { "length": 160.0, "thickness": 16.0 },
+    "triangle": { "base": 56.0, "height": 48.0 },
+    "line":     { "length": 120.0 },
+    "circle":   { "radius": 40.0, "segments": 12 }
+  },
+  "canvas": {
+    "grid_size": 15,
+    "background_color": "#1F1F1F",
+    "grid_color": "#FFFFFF26"
+  },
+  "ui": {
+    "fonts": {
+      "body": 18,
+      "header": 24,
+      "menu": 22,
+      "menu_bar": 22,
+      "mode": 26,
+      "status": 18,
+      "prop_name": 20,
+      "context_menu": 22
+    },
+    "hit_targets": {
+      "button_min_height": 36,
+      "row_min_height": 36,
+      "mode_button_min_height": 56
+    },
+    "colors": {
+      "text": "#F2F2F2",
+      "text_muted": "#9A9A9A",
+      "border": "#FFFFFF33",
+      "accent": "#4FA3FF"
+    }
+  }
+}