FB-03: grid-snapped HUD widgets with auto-fit, centered text

Replace free-form fractional HUD widget positioning with a 4x8 int grid
(col/row/colSpan/rowSpan), reject drag/resize moves that would overlap
another visible widget, and re-place a re-enabled metric into the next
free grid slot instead of stacking it on an occupier. Drag/resize now
track a local pixel offset during the gesture and only snap/commit once
at gesture end. HUD widget text is wrapped in FittedBox(scaleDown) so it
never overflows and stays centered at any grid size.

lib/src/ui/components/hud_edit_overlay.dart

@@ -64,8 +64,9 @@ class _HudEditOverlayState extends ConsumerState<HudEditOverlay> {
                     layout: entry.value,
                     areaSize: areaSize,
                     editing: _editing,
-                    onMoved: (x, y) => notifier.updatePosition(entry.key, x, y),
-                    onResized: (w, h) => notifier.updateSize(entry.key, w, h),
+                    onMoved: (col, row) => notifier.updatePosition(entry.key, col, row),
+                    onResized: (colSpan, rowSpan) =>
+                        notifier.updateSize(entry.key, colSpan, rowSpan),
                     child: widget.metricBuilder(context, entry.key),
                   ),
               if (_editing)