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.

lua/watchexec/config.lua

@@ -69,7 +69,7 @@ local defaults = {
     failure_hl = "WatchexecFailure",
     width = 2,
     height = 1,
-    padding = { x = 1, y = 1 },
+    padding = { x = 1, y = 3 },
     patterns = {
       success = "%[Command was successful%]",
       running = "%[Running",