Overlay: fix 404, crash on repeat, missing edge test colors, device reset on stop

- Target overlay works without active processing: route pre-loads calibration
  and display info from the CSS store, passes to processor as fallback
- Fix server crash on repeated overlay: replace per-window tk.Tk() with single
  persistent hidden root; each overlay is a Toplevel child dispatched via
  root.after() — eliminates Tcl interpreter crashes on Windows
- Fix edge test colors not lighting up: always call set_test_mode regardless
  of processing state (was guarded by 'not proc.is_running'); pass calibration
  so _send_test_pixels knows which LEDs map to which edges
- Fix device reset on overlay stop: keep idle serial client cached after
  clearing test mode; start_processing() already closes it before connecting

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

server/src/wled_controller/api/schemas/color_strip_sources.py

@@ -57,6 +57,7 @@ class ColorStripSourceResponse(BaseModel):
     led_count: int = Field(0, description="Total LED count (0 = auto from calibration)")
     calibration: Optional[Calibration] = Field(None, description="LED calibration")
     description: Optional[str] = Field(None, description="Description")
+    overlay_active: bool = Field(False, description="Whether the screen overlay is currently active")
     created_at: datetime = Field(description="Creation timestamp")
     updated_at: datetime = Field(description="Last update timestamp")