feat(ui): adapt to the screen and its scaling, not to fixed pixels

Two things differ between machines and only one of them is width: a 4K panel
has more pixels, while a 125%/150% display has the same logical pixels holding
LESS, because every label and margin is taller. Breakpoints written as raw
pixels only hold on the machine they were tuned on.

  * ui_scale() derives a factor from font height (1.0 at the 15px line the
    layouts were measured against) and every narrow-guard threshold is
    multiplied by it, so panes fold when the content is cramped rather than
    when a number is crossed.
  * The window takes a share of the available screen (80% × 85%) with the old
    1180×760 as the floor, instead of opening at that size on any monitor.
  * Moving the window to another screen re-pins the assistant and re-decides
    the fold, since the new screen's work area and scaling may differ.

Found by tools/check_multi_screen.py, which walks 5 window sizes × 3 font
scales:

  * At 150%, Schedule was clipped on 1280 and 1366 screens and the window's
    own minimum grew to 1459px — wider than a 1280 laptop, so the app could
    not fit at all. The cause was not the lanes: the one-line lane-count
    summary in the header reported a sizeHint wide enough to set the minimum
    width of the entire window. It now yields first (its text stays in the
    tooltip); the window minimum drops 1459 → 752 and holds there at every
    scale.

Also: these checkers exited 0xC0000409 from a Qt teardown crash AFTER printing
their verdict. check_probes_bite decides whether a probe caught its mutation by
reading exit codes, so a crash would have counted as "caught" — the round could
have passed while proving nothing. They now flush and os._exit with the real
verdict, and round 5 still catches all six mutations.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/check_dialogs.py

@@ -127,4 +127,11 @@ def main() -> int:
 
 
 if __name__ == "__main__":
-    raise SystemExit(main())
+    _rc = main()
+    # Qt (WebEngine especially) crashes during interpreter teardown with
+    # 0xC0000409 AFTER the work is done, which would mask the real result —
+    # and check_probes_bite reads these exit codes to decide whether a probe
+    # caught its mutation. Leave immediately with the verdict instead.
+    sys.stdout.flush()
+    sys.stderr.flush()
+    os._exit(_rc)