fix: 更新所有图标为 JRD

- 任务栏图标使用 nativeImage
- Linux 使用 app.setIcon()
- TitleBar logo 改为 JRD
- MainArea placeholder 改为 JRD

src/renderer/src/components/MainArea.vue

@@ -77,7 +77,7 @@ watch(
   </div>
 
   <div v-else class="no-connection">
-    <div class="placeholder-icon">R</div>
+    <div class="placeholder-icon">JRD</div>
     <h2>JRedisDesktop</h2>
     <p>Select or create a connection to get started</p>
   </div>
@@ -137,18 +137,19 @@ watch(
 }
 
 .placeholder-icon {
-  width: 64px;
+  width: 80px;
   height: 64px;
   background: linear-gradient(135deg, #667eea, #764ba2);
   border-radius: 16px;
   display: flex;
   align-items: center;
   justify-content: center;
-  font-size: 28px;
+  font-size: 22px;
   font-weight: 800;
   color: #fff;
   margin-bottom: 20px;
   box-shadow: 0 8px 32px rgba(99, 102, 241, 0.2);
+  letter-spacing: 1px;
 }
 
 .no-connection h2 {

src/renderer/src/components/TitleBar.vue

@@ -15,7 +15,7 @@ function toggleTheme() {
 <template>
   <header class="titlebar">
     <div class="titlebar-drag">
-      <span class="titlebar-logo">R</span>
+      <span class="titlebar-logo">JRD</span>
       <span class="titlebar-title">JRedisDesktop</span>
     </div>
     <div class="titlebar-actions">
@@ -50,16 +50,17 @@ function toggleTheme() {
 }
 
 .titlebar-logo {
-  width: 24px;
+  width: 36px;
   height: 24px;
   background: linear-gradient(135deg, #667eea, #764ba2);
   border-radius: 6px;
   display: flex;
   align-items: center;
   justify-content: center;
-  font-size: 12px;
-  font-weight: 700;
+  font-size: 9px;
+  font-weight: 800;
   color: #fff;
+  letter-spacing: 0.5px;
 }
 
 .titlebar-title {