v0.27.3: Use real system memory everywhere, add monitoring memory bar

Deploy page, pre-start check, and deploy validation now use actual
/proc/meminfo usage instead of declared mem_request sums. New
GetMemoryMB() helper for lightweight real-time memory reads. Monitoring
page gains a stacked memory distribution bar showing per-container
usage, OS overhead, and free memory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/system/info_linux.go

@@ -54,6 +54,16 @@ func GetTotalMemoryMB() (int, error) {
 	return int(info.TotalMemMB), nil
 }
 
+// GetMemoryMB returns total and used system memory in MB from /proc/meminfo.
+func GetMemoryMB() (totalMB, usedMB int, err error) {
+	info := SystemInfo{}
+	readMemInfo(&info)
+	if info.TotalMemMB == 0 {
+		return 0, 0, fmt.Errorf("could not read MemTotal from /proc/meminfo")
+	}
+	return int(info.TotalMemMB), int(info.UsedMemMB), nil
+}
+
 func readMemInfo(info *SystemInfo) {
 	f, err := os.Open("/proc/meminfo")
 	if err != nil {