feat: v0 tab completion

[wjiayis]

#33

In short, it'll

1. [Frontend] Recognize that user is trying to add a citation (trigger is \cite{)
2. [Frontend] Suppress default overleaf \cite{} dropdown suggestions
3. [Frontend] Get the last sentence as context for LLM
4. [Backend] Fetch bibliography in .bib files stored in project as raw text
5. [Backend] Query a lightweight LLM (hardcoded to gpt-5-nano for now) to get at most 3 citation keys

There are some outstanding issues

1. Latency is way too high (severe) -> I'll see what kind of caching / design choices I can do to bring the latency down
2. When user is idle for a long time, token expires and user is logged out. User will not see citation suggestions until they log back in -> won't fix in this iteration