refactor: absorb statusline offset into indicator padding

Remove the hardcoded statusline_offset from calculate_position() and
let padding.y serve as the sole offset from the bottom edge. Bump
default padding.y to 3 to preserve the original visual position.

doc/watchexec.txt

@@ -187,7 +187,7 @@ indicator (table|nil) ~
 
     y (integer|nil) ~
       Vertical offset from the top or bottom edge.
-      Default: 1
+      Default: 3
 
   patterns (table|nil) ~
     Lua patterns for detecting command lifecycle in output.