Fix welcome dashboard jitter and refine Morse mode UI

Fix "What's New" section shifting up/down on smaller screens (#157) by
isolating the logo pulse animation to its own compositing layer, stabilizing
the scrollbar gutter, and pinning the welcome container dimensions.

Morse mode improvements: relocate scope and decoded output panels to the
main content area, use shared SDR device controls, and reduce panel heights
for better layout.

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

static/css/index.css

@@ -206,6 +206,8 @@ body {
     max-width: 900px;
     z-index: 1;
     animation: welcomeFadeIn 0.8s ease-out;
+    max-height: calc(100vh - 40px);
+    overflow: hidden;
 }
 
 @keyframes welcomeFadeIn {
@@ -232,6 +234,7 @@ body {
 
 .welcome-logo {
     animation: logoPulse 3s ease-in-out infinite;
+    will-change: filter;
 }
 
 @keyframes logoPulse {
@@ -332,6 +335,7 @@ body {
     padding: 20px;
     max-height: calc(100vh - 300px);
     overflow-y: auto;
+    scrollbar-gutter: stable;
 }
 
 .changelog-release {