CRITICAL FIX: Enable renderer fallbacks for Windows compatibility

Root Cause Analysis:
- export_presets.cfg had ANGLE=0 and D3D12=0 (DISABLED!)
- Windows users without Vulkan support had NO renderer fallback
- This caused immediate freeze/hang on startup for many Windows PCs

Fixes applied:

1. export_presets.cfg:
   - Enable ANGLE (OpenGL ES via DirectX 11): export_angle=2
   - Enable D3D12 fallback: export_d3d12=1
   - This provides fallback renderers for systems without Vulkan

2. scene_transition_manager.gd:
   - Replace synchronous load() with threaded ResourceLoader
   - Add 30-second timeout to prevent infinite hangs
   - Add fallback to change_scene_to_file on failure

3. ShiftSummaryScreen.gd (line 958-982):
   - Add 30-second timeout to while-loop resource loading
   - Handle THREAD_LOAD_INVALID_RESOURCE status
   - Fallback to direct scene load on timeout

4. simple_scene_loader.gd:
   - Add 60-second timeout to threaded load loop
   - Handle THREAD_LOAD_INVALID_RESOURCE status
   - Prevent infinite loop in background thread

Additional findings for future investigation:
- Multiple autoloads call Steam.isSteamRunning() during _ready()
  which could block if Steam is unresponsive (Global.gd:68,
  SteamManager.gd:51, SentryManager.gd:77, LogManager.gd:52)