Added carbonation drops to fermenting bucket rather than when bottling. Have I ruined my brew?
No you haven't ruined the beer, you have two options:
Bottle without adding extra sugar. This will result in slightly under carbonated beer as the drops won't fully dissolve in the fermenter before bottling.
Allow fermentation to finish out in the fermenter again and try bottling again using the carbonation drops following the instructions. The resulting beer will be more alcoholic and have slightly less body but will otherwise be fine.