feat: implement reliable async analytics delivery

[coderabbitai]

Summary

Currently, postRunAnalytics and the root cli.PersistentPostRun use fire-and-forget goroutines (go analytics.PostRunAnalytics(...)) without any synchronization. This is an intentional trade-off — blocking on analytics network I/O made fast CLI commands feel slow — but it means analytics events can be silently dropped when the process exits before the goroutine runs.

Goal

Explore a non-blocking but more reliable delivery mechanism, for example:

• Buffering events to a local file/queue and flushing them on the next CLI invocation.
• Using a short, bounded timeout (e.g., 200–500 ms) to allow the goroutine to finish without meaningfully impacting UX.
• Any other approach that preserves CLI responsiveness while reducing event loss.

Context

• Raised during review of PR feat: refine CLI workflows for datasets, queries, auth, and help #100 (comment: feat: refine CLI workflows for datasets, queries, auth, and help #100 (comment))
• Decision to keep fire-and-forget for now was made by @pratik50 to avoid blocking fast commands.

Acceptance Criteria

• Analytics goroutines do not cause noticeable latency increase for fast CLI commands.
• Event loss rate is reduced compared to the current best-effort approach.