Fix raw debug output for negative estimated end durations

[rootkiller6788]

Fixes #761.

Problem

On the experiment page, the Estimated end field sometimes showed the raw debug representation of a chrono::Duration, e.g.:

Estimated end:  TimeDelta { secs: -1, nanos: 884994194 }

Root cause

In src/server/routes/ui/experiments.rs, endpoint_experiment computes the estimated remaining time as:

job_duration * (total_jobs as i32 - completed_jobs as i32)

total_jobs is crates_len * 2 while completed_jobs is the count of rows in the results table. Retried or extra result rows can make completed_jobs exceed total_jobs, producing a negative duration. chrono::Duration::to_std() rejects negative values, so humanize() fell through to format!("{duration:?}"), leaking the internal TimeDelta representation into the UI.

Fix

humanize() now reports a negative (unrepresentable) duration as 0 seconds instead of the raw debug output.

Verification

• Confirmed raw_progress returns (results_len, crates_len * 2), so completed_jobs > total_jobs is reachable when extra result rows exist (e.g. retried jobs).
• Confirmed chrono 0.4.45 Duration (a TimeDelta alias) debug-prints as TimeDelta { secs: ..., nanos: ... }, matching the screenshot in the issue.

[oli-obk]

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