feat(map): show first-call labels as soon as grid lines appear

Move the first-call label gate from LABEL_ZOOM_SUB (6.5) to GRID_ZOOM_SUB
(6.0): worked (green) cells are labelled with their first callsign as soon
as the 4-char grid lines render, not only at the name-label zoom.

feature/map/src/main/java/com/rtbishop/look4sat/feature/map/MaidenheadGridOverlay.kt

@@ -367,7 +367,9 @@ class MaidenheadGridOverlay : Overlay() {
         // First-call labels replace the 4-char grid codes and only exist at
         // sub-square zoom; at field zoom NO labels are drawn at all (the user
         // requirement is that non-worked cells carry no grid characters).
-        if (showFirstCallLabels && zoom < LABEL_ZOOM_SUB) return
+        // Show as soon as the 4-char grid LINES appear (GRID_ZOOM_SUB), not
+        // only at the name-label zoom — user req 2026-09-21.
+        if (showFirstCallLabels && zoom < GRID_ZOOM_SUB) return
         val showLabels = zoom >= LABEL_ZOOM_SUB || cellLat == FIELD_LAT
         if (!showLabels) return
         // Estimate on-screen cell height to avoid clutter at low zoom: