fix: add undo support for autocomplete suggestions

fix(autocomplete): only trigger after space - no partial words

feat(autocomplete): threshold=0.05 + max 3 suggestions

fix(autocomplete): 3 user-reported issues - last word only, dropdown position, cursor after accept

fix(autocomplete): fix dropdown not showing - 3 critical bugs fixed

feat(NLP-4): AutoComplete integration — hybrid bigram + GPT-2