feat(logs): URL deep-link + fetched-age chip for AGENT/SYSTEM tabs

Adds ?agent=name and ?unit=svc URL params to pre-select the agent on
the AGENT tab (used by the per-agent overflow menu journal logs link).
Last-fetched timestamp chip shows how stale the output is.

Agent selection is NOT persisted to localStorage — URL params only.

frontend/packages/dashboard/src/logs.css

@@ -71,6 +71,8 @@ body.logs-shell {
   font-size: 0.85em;
 }
 .journal-refresh { font-size: 0.75em; padding: 0.15em 0.5em; }
+/* "fetched N ago" chip next to the refresh button on AGENT + SYSTEM tabs. */
+.logs-fetch-ts { font-size: 0.8em; color: var(--muted); }
 .journal-output {
   margin: 0;
   background: var(--crust);