feat(#986): dedicated logs page with build/agent/system sub-tabs

Add /logs.html as a standalone page (same back-link pattern as flow.html):
- BUILD tab: all-agents build log history via new GET /api/build-logs endpoint
- AGENT tab: per-container journald viewer with agent selector + unit filter
- SYSTEM tab: host-side hive-c0re.service logs via new GET /api/journal-host endpoint

Remove inline log drill-ins from SW4RM container rows (buildJournalTrigger
and buildBuildLogsTrigger) — log viewing now lives on the dedicated page.

flow.html: strip the full dashboard tabbar, replace with a simple back link
matching the new logs page chrome.

index.html: add L0GS tab link to /logs.html in the tab strip.

Backend additions:
- build_logs::list_recent_all — cross-agent query (newest first, cap 100)
- GET /api/build-logs — all-agents variant backed by list_recent_all
- GET /api/journal-host — host journald (no -M container flag), restricted
  to allow-listed units (hive-c0re.service)

frontend/packages/dashboard/src/dashboard.css

@@ -1856,6 +1856,94 @@ body.flow-shell .tabbar .tab.active.tab-link {
    surface the messages via the pill/flyout instead. */
 .flow-inbox-headless { display: none !important; }
 
+/* ─── /logs.html — log viewer ──────────────────────────────────────
+   Same minimal-chrome pattern as /flow.html but with a sub-tab strip
+   instead of a full dashboard tabbar. */
+
+body.logs-shell {
+  margin: 0;
+  padding: 0;
+}
+body.flow-shell .flow-main-slim {
+  padding-top: calc(var(--flow-header-h) + 0.4em);
+}
+
+.logs-header {
+  position: sticky;
+  top: 0;
+  z-index: 25;
+  background: rgba(30, 30, 46, 0.92);
+  -webkit-backdrop-filter: blur(8px) saturate(120%);
+  backdrop-filter: blur(8px) saturate(120%);
+  border-bottom: 1px solid var(--purple-dim);
+  display: flex;
+  align-items: center;
+  gap: 1.5em;
+  padding: 0.5em 1.5em;
+}
+
+.logs-back {
+  color: var(--mauve);
+  text-decoration: none;
+  font-size: 0.88em;
+  white-space: nowrap;
+  flex: none;
+}
+.logs-back:hover { text-decoration: underline; }
+
+.logs-title {
+  color: var(--subtext0);
+  font-size: 0.85em;
+  letter-spacing: 0.05em;
+  flex: none;
+}
+
+.logs-tabbar {
+  display: flex;
+  gap: 0.2em;
+  flex: 1;
+}
+
+.logs-tab {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.35em 0.9em;
+  border-radius: 4px;
+  color: var(--subtext0);
+  text-decoration: none;
+  font-size: 0.85em;
+  letter-spacing: 0.05em;
+  transition: background 100ms, color 100ms;
+}
+.logs-tab:hover { background: var(--surface1); color: var(--text); }
+.logs-tab.logs-tab-active {
+  background: var(--surface1);
+  color: var(--mauve);
+}
+
+.logs-main {
+  padding: 1.2em 1.5em 2em;
+}
+
+.logs-pane[hidden] { display: none; }
+
+.logs-toolbar {
+  display: flex;
+  gap: 0.6em;
+  align-items: center;
+  margin-bottom: 0.9em;
+  flex-wrap: wrap;
+}
+
+.logs-toolbar .journal-unit {
+  background: var(--surface1);
+  color: var(--text);
+  border: 1px solid var(--surface2);
+  border-radius: 4px;
+  padding: 0.25em 0.5em;
+  font-size: 0.85em;
+}
+
 /* ─── scheduled prompts tab ────────────────────────────────────────
    Creation form at the top, list of queued schedule cards below.
    Cards show: id + source + due-in + cancel-all in the header,