Hover the feature card only for installable plugins

The hover on the plugins' feature cards should generate a transition and
shadow only for those plugins that aren't already installed. The
animation and raising of the card should have a visual experience of
inviting the admin to experiment with the plugin and install it.

Before this change, all the feature cards were animated, and the
hovering had no real meaning beyond its visual effect.

Change-Id: I813560163bb869640d6c523ddec1bf6a3cccdfb4

src/main/resources/static/css/intro.css

diff --git a/src/main/resources/static/css/intro.css b/src/main/resources/static/css/intro.css
index 04195c0..88c41cd 100644
--- a/src/main/resources/static/css/intro.css
+++ b/src/main/resources/static/css/intro.css
@@ -151,7 +151,6 @@
 .install-btn:hover {
     background-color: var(--primary-hover);
     box-shadow: 0 10px 15px -3px rgba(37, 99, 235, 0.4);
-    transform: translateY(-2px);
 }
 
 .features-section {
@@ -174,9 +173,9 @@
     transition: transform 0.2s ease, box-shadow 0.2s ease;
 }
 
-.feature-card:hover {
+.feature-card.card-installable:hover {
     transform: translateY(-4px);
-    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1);
+    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.5);
 }
 
 .feature-title {