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_other.go

@@ -13,3 +13,8 @@ func GetInfo(_ string, _ *CPUCollector) SystemInfo {
 func GetTotalMemoryMB() (int, error) {
 	return 0, fmt.Errorf("/proc/meminfo not available on this platform")
 }
+
+// GetMemoryMB is not available on non-Linux platforms.
+func GetMemoryMB() (totalMB, usedMB int, err error) {
+	return 0, 0, fmt.Errorf("/proc/meminfo not available on this platform")
+}