feat: add system metrics dashboard with resource monitoring

- Add system metrics endpoint (CPU load, memory, swap, network traffic)
- Collect metrics from /proc and /sys (no subprocess required)
- Overhaul dashboard to pull from per-subsystem models
- Remove deprecated /status/all monolithic endpoint
- Improve networkd import to handle optional priority prefix
- Fix CSS duplicate .grid-4 rule and unused dashboard imports

lib/state.py

@@ -36,6 +36,7 @@ _DEFAULT_POLL_INTERVALS: dict[str, int] = {
     "wireguard": 10,
     "dnsmasq": 10,
     "networkd": 10,
+    "system": 30,
 }
 
 
@@ -63,6 +64,7 @@ class State:
         "acme",
         "wireguard",
         "networkd",
+        "system",
     ]
 
     def __init__(self) -> None:
@@ -1071,6 +1073,118 @@ register_volatile(
     ),
 )
 
+# ---------------------------------------------------------------------------
+# System metrics collector
+# ---------------------------------------------------------------------------
+
+
+def _parse_meminfo() -> dict[str, Any]:
+    """Read /proc/meminfo and return dict with key memory stats in bytes."""
+    info: dict[str, int] = {}
+    try:
+        for line in Path("/proc/meminfo").read_text().splitlines():
+            if ":" not in line:
+                continue
+            key, value = line.split(":", 1)
+            key = key.strip()
+            parts = value.strip().split()
+            val = int(parts[0])
+            # Convert kB to bytes
+            if parts and parts[-1] == "kB":
+                val *= 1024
+            info[key] = val
+    except (OSError, ValueError):
+        return {}
+    return info
+
+
+def _collect_system() -> dict[str, Any]:
+    """Collect system-wide metrics: CPU load, memory, network traffic.
+
+    Reads from /proc and /sys — no subprocess needed.
+
+    Returns:
+        Dict with load (1/5/15 min), memory usage, and per-interface traffic.
+    """
+    # CPU load
+    loads = []
+    try:
+        parts = Path("/proc/loadavg").read_text().split()
+        loads = [float(x) for x in parts[:3]]
+    except (OSError, ValueError):
+        loads = [0.0, 0.0, 0.0]
+
+    # Memory
+    meminfo_raw = _parse_meminfo()
+    mem_total = meminfo_raw.get("MemTotal", 0)
+    mem_free = meminfo_raw.get("MemFree", 0)
+    mem_available = meminfo_raw.get("MemAvailable", mem_free)
+    mem_buffers = meminfo_raw.get("Buffers", 0)
+    mem_cached = meminfo_raw.get("Cached", 0)
+    mem_used = mem_total - mem_free - mem_buffers - mem_cached
+    if mem_used < 0:
+        mem_used = mem_total - mem_available
+
+    # Swap
+    swap_total = meminfo_raw.get("SwapTotal", 0)
+    swap_free = meminfo_raw.get("SwapFree", 0)
+    swap_used = swap_total - swap_free
+
+    # Network traffic from /sys/class/net/<iface>/statistics/
+    traffic: dict[str, dict[str, int]] = {}
+    try:
+        net_root = Path("/sys/class/net")
+        if net_root.is_dir():
+            for iface_dir in net_root.iterdir():
+                stats_dir = iface_dir / "statistics"
+                if not stats_dir.is_dir():
+                    continue
+                iface_name = iface_dir.name
+                rx_bytes = 0
+                tx_bytes = 0
+                rx_packets = 0
+                tx_packets = 0
+                try:
+                    rx_bytes = int((stats_dir / "rx_bytes").read_text().strip())
+                    tx_bytes = int((stats_dir / "tx_bytes").read_text().strip())
+                    rx_packets = int((stats_dir / "rx_packets").read_text().strip())
+                    tx_packets = int((stats_dir / "tx_packets").read_text().strip())
+                except (OSError, ValueError):
+                    continue
+                traffic[iface_name] = {
+                    "rx_bytes": rx_bytes,
+                    "tx_bytes": tx_bytes,
+                    "rx_packets": rx_packets,
+                    "tx_packets": tx_packets,
+                }
+    except OSError:
+        pass
+
+    return {
+        "load": {
+            "load1": loads[0],
+            "load5": loads[1],
+            "load15": loads[2],
+        },
+        "memory": {
+            "total": mem_total,
+            "available": mem_available,
+            "used": mem_used,
+            "used_pct": round(mem_used / mem_total * 100, 1) if mem_total > 0 else 0,
+        },
+        "swap": {
+            "total": swap_total,
+            "used": swap_used,
+            "used_pct": round(swap_used / swap_total * 100, 1) if swap_total > 0 else 0,
+        },
+        "traffic": traffic,
+        "timestamp": _now_iso(),
+    }
+
+
+register_collector("system", _collect_system)
+
+
 __all__ = [
     "_DEFAULT_POLL_INTERVALS",
     "State",