feat(daq): compute beam size for the mark overlay (rectangle, not crosshair)

Fill in the beam SIZE in save_screenshot_db using the same conversion the
raster grid uses: beam_size_mm / pixel_to_mm(zoom) (sample-camera mm/px).
The frontend now draws the beam rectangle (matching the gridscan) instead
of a centre-only crosshair. Best-effort; falls back to crosshair on error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/aare/daq/daq.py

@@ -2506,18 +2506,19 @@ class AareDAQ:
             filename: Name to give to the uploaded image.
             settle_time_s: float time to wait before taking the screenshot.
         """
-        # Beam-mark centre in sample-camera pixels at the current zoom, for the
-        # DB overlay (drawn in the frontend, never baked into the JPEG). Best
-        # effort — a calibration hiccup must never block the screenshot upload.
+        # Beam-mark centre + size in sample-camera pixels at the current zoom,
+        # for the DB overlay (drawn in the frontend, never baked into the JPEG).
+        # Best effort — a calibration hiccup must never block the screenshot.
         beam_mark_pxl = None
         beam_size_pxl = None
         try:
             beam_mark_pxl = self.get_beam_mark()
-            # TODO(calibration): beam SIZE in sample-camera pixels so the overlay
-            # is a rectangle instead of a crosshair, e.g.
-            #     px_per_mm = ...  # sample-camera scale at self.__devs.zoom
-            #     bs = self.__cfg.beam_size_mm
-            #     beam_size_pxl = (bs.x * px_per_mm, bs.y * px_per_mm)
+            # Same conversion the raster grid uses (size_mm / pixel_in_mm);
+            # pixel_to_mm(zoom) is the sample-camera scale in mm per pixel.
+            pixel_in_mm = self.__cfg.pixel_to_mm(self.__devs.zoom)
+            if pixel_in_mm and pixel_in_mm > 0:
+                bs = self.__cfg.beam_size_mm
+                beam_size_pxl = (bs.x / pixel_in_mm, bs.y / pixel_in_mm)
         except Exception:
             logger.warning("Could not read beam mark for screenshot overlay", exc_info=True)