feat(map): tap worked cells from GRID_ZOOM_SUB (lines visible), labels from zoom 6.5

- Tap-to-show-QSOs now works as soon as 4-char grid lines appear (zoom 6),
  not only when 4-char labels show (was zoom 7). Selection outline already
  used GRID_ZOOM_SUB, so feedback is immediate.
- 4-char labels now appear from zoom 6.5 (was 7.0), so the sub-square name
  shows shortly after the lines appear instead of a full zoom level later.

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

@@ -415,8 +415,8 @@ class MaidenheadGridOverlay : Overlay() {
         // on-screen size of each square.
         const val GRID_ZOOM_SUB = 6.0
         // Zoom at which the 4-char sub-square names appear (one level above the
-        // grid lines: zoom 6 = lines only, zoom 7+ = lines + names).
-        const val LABEL_ZOOM_SUB = 7.0
+        // grid lines: zoom 6 = lines only, zoom 6.5+ = lines + names).
+        const val LABEL_ZOOM_SUB = 6.5
         const val MIN_LABEL_CELL_PX = 48f
         const val MAX_OVERSHOOT_PX = 64
     }

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

@@ -153,7 +153,9 @@ private fun MapScreen(uiState: MapState, onAction: (MapAction) -> Unit, mapView:
             override fun singleTapConfirmedHelper(p: GeoPoint?): Boolean {
                 if (!isGridMode || p == null) return false
                 val zoom = mapView.zoomLevelDouble
-                if (zoom < MaidenheadGridOverlay.LABEL_ZOOM_SUB) return false
+                // Allow tapping worked cells as soon as the 4-char grid LINES
+                // appear (GRID_ZOOM_SUB), not only when labels show (LABEL_ZOOM_SUB).
+                if (zoom < MaidenheadGridOverlay.GRID_ZOOM_SUB) return false
                 val grid = gridOfPoint(p.latitude, p.longitude) ?: return false
                 if (grid !in workedGrids) return false
                 selectedGrid = grid