builds: name the failed step and duration; jump to failure on CLI and web #266

A failed build does not say which step failed or how long it ran.

Build 1510 (test, failure): the log is 1,363 lines, the failure (--- FAIL: TestCLI) is at line 1049, and the last line is step failed: exit status 1 with no step named.

• CLI: build show prints sha, ref, queued, finished and url. No duration, failed step or exit status. build log has only --follow and --json.
• Web: build.html:16-17 renders one <pre> with no step structure or anchor. pre.buildlog { max-height: 40rem; overflow: auto; } (style.css:1116) cuts long lines at phone width with no visible scroll cue.
• iOS: filed separately in krz/gitbay-ios (the log renders blank).

Changes:

• Runner: step 3/3 failed: exit status 1, with the step number.
• Store the failed step and duration on the build; build show prints failed step 3/3 go test ./... (exit 1) and duration 10m56s.
• build log --step <n> and --tail <n>.
• Web: one <details> per step with the failed one open, id="failed" and a "Jump to failure" link, duration next to "finished". No JavaScript needed.
• pre.buildlog: white-space: pre-wrap; overflow-wrap: anywhere, as pre.message has.