Add frame-change detection, keepalive, current FPS, and compact metrics UI

Skip redundant processing/DDP sends when screen is static using object
identity comparison. Add configurable standby interval to periodically
resend last frame keeping WLED in live mode. Track frames skipped,
keepalive count, and current FPS (rolling 1-second send count). Always
use DDP regardless of LED count. Compact metrics grid with label-value
rows and remove Skipped from UI display.

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

server/src/wled_controller/static/style.css

@@ -844,28 +844,29 @@ input:-webkit-autofill:focus {
 
 .metrics-grid {
     display: grid;
-    grid-template-columns: repeat(2, 1fr);
-    gap: 10px;
-    margin-top: 10px;
+    grid-template-columns: 1fr 1fr;
+    gap: 4px 12px;
+    margin-top: 8px;
 }
 
 .metric {
-    text-align: center;
-    padding: 10px;
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 3px 8px;
     background: var(--bg-color);
     border-radius: 4px;
 }
 
 .metric-value {
-    font-size: 1.5rem;
+    font-size: 0.9rem;
     font-weight: 700;
     color: var(--primary-color);
 }
 
 .metric-label {
-    font-size: 0.85rem;
+    font-size: 0.8rem;
     color: #999;
-    margin-top: 5px;
 }
 
 /* Modal Styles */