style: reduce vertical spacing in medium-sized bookmark cards

- Reduce reading progress separator margin from 0.75rem to 0.5rem
- Reduce card content gap from 0.75rem to 0.5rem
- Reduce bookmark title margin-bottom from 0.75rem to 0.5rem
- Reduce bookmark footer padding-top from 0.75rem to 0.5rem
- Update responsive breakpoints for consistent compact spacing
- Make cards more compact and visually tighter

src/styles/components/cards.css

@@ -160,7 +160,7 @@
 .card-content {
   display: flex;
   flex-direction: column;
-  gap: 0.75rem;
+  gap: 0.5rem;
   flex: 1;
 }
 
@@ -172,7 +172,7 @@
   font-size: 1.1rem;
   font-weight: 600;
   color: var(--color-text);
-  margin: 0 0 0.75rem 0;
+  margin: 0 0 0.5rem 0;
   line-height: 1.4;
   overflow: hidden;
   text-overflow: ellipsis;
@@ -200,7 +200,7 @@
 
 .card-content .bookmark-footer {
   margin-top: auto;
-  padding-top: 0.75rem;
+  padding-top: 0.5rem;
   display: flex;
   justify-content: space-between;
   align-items: center;
@@ -238,7 +238,7 @@
   background: var(--color-border);
   border-radius: 0.5px;
   overflow: hidden;
-  margin: 0.75rem 0;
+  margin: 0.5rem 0;
   position: relative;
 }
 
@@ -314,7 +314,7 @@
   
   .card-content .bookmark-title {
     font-size: 1rem;
-    margin-bottom: 0.5rem;
+    margin-bottom: 0.375rem;
   }
   
   .card-content .bookmark-content {
@@ -323,7 +323,7 @@
   }
   
   .card-content .bookmark-footer {
-    padding-top: 0.5rem;
+    padding-top: 0.375rem;
   }
 }
 
@@ -348,7 +348,7 @@
   
   .card-content .bookmark-title {
     font-size: 0.9rem;
-    margin-bottom: 0.5rem;
+    margin-bottom: 0.375rem;
   }
   
   .card-content .bookmark-content {