chore: frontend cleanup — replaceChildren, missing source badge, http case

- Replace innerHTML = '' with replaceChildren() throughout tabs.js and
  app.js (12 + 7 sites). paintAtomic already used replaceChildren; now
  the direct-clear sites are consistent with it.
- Add missing .rqe-source-startup_sweep CSS rule (startup_sweep is a
  valid QueueSource variant but had no badge style, falling through to
  the base muted appearance with no explicit intent).
- Lowercase the one uppercase 'HTTP ' in the fetchAndRenderToolGroups
  error path to match every other fetch error in the file.

frontend/packages/dashboard/src/dashboard.css

@@ -1096,6 +1096,7 @@ code {
 .rqe-source-manual { color: var(--cyan); border-color: var(--cyan); }
 .rqe-source-meta_update { color: var(--purple); border-color: var(--purple); }
 .rqe-source-auto_update { color: var(--muted); }
+.rqe-source-startup_sweep { color: var(--muted); }
 .rqe-source-crash_recover { color: var(--amber); border-color: var(--amber); }
 .rqe-source-approval { color: var(--green); border-color: var(--green); }
 .rqe-when { color: var(--muted); font-size: 0.85em; }