dashboard: consolidate NodeView.has_log into build_log_id

Per mara's review on #2896: has_log: bool was fully redundant once
build_log_id: Option<i64> existed alongside it (has_log was always
just build_log_id.is_some()). Dropped has_log, threading the single
Option<i64> field through job_queue::mod.rs, the hivectl NodeView
test-helper literal, and the one remaining frontend consumer
(findLiveBuild's live-log-panel gate, which now checks
build_log_id != null instead of the separate bool).

Also fixed a now-stale doc comment on GET /api/build-log/{node_id}
that claimed the dashboard used on-demand node-id fetches "instead
of an inline build_log_id on the wire" -- no longer true after this
PR put one there for the BUILD L0GS deep-link.

cargo build/clippy/test clean across the three touched crates; nix
fmt clean; frontend build verified (0 has_log references, 3
build_log_id references in the built builds.js bundle).

hivectl/src/dag_progress.rs

@@ -342,7 +342,6 @@ mod tests {
             error: None,
             approval_id: None,
             inputs: vec![],
-            has_log: false,
             build_log_id: None,
         }
     }