Dashboard: per-host resource overview bar (0.20.0)

The dashboard resource bar (CPU cores, memory used/total, containers, Docker
version) is now rendered per host instead of once for the local host — each
host section (local + each agent) shows its own bar above its stacks table.

- agent_app.py: _system_info() now also returns mem_used (from meminfo
  available), alongside the cpu_cores/mem_total added in 0.19.0.
- Frontend: extracted a ResourceBar component used by the local section and each
  AgentDashboardSection; AgentSystem type gained mem_used.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/agent_app.py

@@ -61,7 +61,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.19.0"
+AGENT_VERSION = "0.20.0"
 
 
 # --------------------------------------------------------------------------- #
@@ -159,16 +159,25 @@ def _hostname() -> str:
     return os.uname().nodename
 
 
-def _mem_total() -> int:
+def _mem_info() -> tuple[int, int]:
+    """Return (total_bytes, used_bytes) from meminfo (used = total - available)."""
     for base in (settings.HOST_PROC_PATH, "/proc"):
         try:
+            vals: dict[str, int] = {}
             with open(os.path.join(base, "meminfo"), "r", encoding="utf-8") as fh:
                 for line in fh:
-                    if line.startswith("MemTotal:"):
-                        return int(line.split()[1]) * 1024  # kB -> bytes
+                    parts = line.split(":")
+                    if len(parts) == 2 and parts[0] in ("MemTotal", "MemAvailable", "MemFree"):
+                        try:
+                            vals[parts[0]] = int(parts[1].split()[0]) * 1024  # kB -> bytes
+                        except ValueError:
+                            pass
+            total = vals.get("MemTotal", 0)
+            available = vals.get("MemAvailable", vals.get("MemFree", 0))
+            return total, max(total - available, 0)
         except OSError:
             continue
-    return 0
+    return 0, 0
 
 
 def _system_info() -> dict:
@@ -184,12 +193,14 @@ def _system_info() -> dict:
         total = info.get("Containers", 0)
     except DockerError as exc:
         docker_version = f"unavailable ({exc.error})"
+    mem_total, mem_used = _mem_info()
     return {
         "hostname": _hostname(),
         "docker_version": docker_version,
         "host_os": host_os,
         "cpu_cores": os.cpu_count() or 0,
-        "mem_total": _mem_total(),
+        "mem_total": mem_total,
+        "mem_used": mem_used,
         "containers_running": running,
         "containers_total": total,
     }