fix(flomni): clarify x-ray eye alignment height-step instruction

The height-marking step's GUI message just said "Submit height. Use
arrows if far off.", which didn't tell users they first need to mark
the position with the image's selection (ROI) tool before submitting.
Spell that out explicitly. Verified against message_line_edit's actual
90px-tall QTextEdit box (across a 140-300px width range) that the
longer text still wraps within the fixed height, no scrollbar needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0197yD3hTxwvrgmYr7aaYiCD

csaxs_bec/bec_ipython_client/plugins/flomni/x_ray_eye_align.py

@@ -299,7 +299,7 @@ class XrayEyeAlign:
                     self.gui.show_crosshair()
 
                     self.send_message(
-                        "Submit height. Use arrows if far off."
+                        "Mark the sample height with the selection tool, then submit. Use arrows if far off."
                     )
                     self.gui.enable_submit_button(True)
                     self.movement_buttons_enabled(True, True)