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/index.html

@@ -71,6 +71,14 @@
         <span class="tab-label">◆ M4TR1X ◆ →</span>
       </a>
 
+      <!-- L0GS is its own page (`/logs.html`). Links to the dedicated
+           log viewer with three sub-tabs (build history, agent journal,
+           system services). Same nav pattern as FL0W. -->
+      <a class="tab tab-link" id="tab-logs" href="/logs.html"
+         title="open the log viewer">
+        <span class="tab-label">◆ L0GS ◆ →</span>
+      </a>
+
       <!-- FL0W is its own page (`/flow.html`), not a tab. The link
            lives in the tab strip so it reads as a peer surface;
            clicking navigates rather than swapping panes in place.