feat(statusbar): honor $COORDS in the coordinate readout, click to cycle

The Coords pill always showed live absolute X,Y,Z, ignoring the drawing's
$COORDS (coords_mode) sysvar. Drive the readout from it:

- 0 static  — the last picked point; the readout freezes between picks.
- 1 live    — continuous absolute X,Y,Z (previous behavior, default).
- 2 polar   — distance<angle relative to the last point while a command is
              prompting for a point; absolute otherwise.

The cursor and last point are reported in the active UCS, matching the rest of
the readout. Clicking the pill cycles the mode 0→1→2→0 (CycleCoordsMode), marks
the tab dirty and echoes it; SETVAR COORDS still works. $COORDS round-trips per
drawing via acadrust's coords_mode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1458,6 +1458,8 @@ pub enum Message {
     ToggleLayoutList,
     /// Close the Model/layout list dropdown.
     CloseLayoutList,
+    /// Cycle the coordinate readout mode ($COORDS): static → live → polar.
+    CycleCoordsMode,
     /// Toggle the status-bar customization menu open/closed.
     ToggleStatusBarMenu,
     /// Close the status-bar customization menu.