fix(examples): fix resize handle alignment and thread border gap in RTL (#3091)

### 🎯 Goal

Fix the "double vertical line" visual artifact on the resize handle in
RTL layouts.

### 🛠 Implementation details

- **Resize handle hitbox**: replaced `translateX(-50%)` centering with
`inset-inline-start: -6px` so the hitbox aligns correctly over the 1px
handle in both LTR and RTL
- **Thread border**: set `--str-chat__thread-border-inline-start: none`
on the thread container when preceded by the resize handle, preventing a
double-line artifact where the handle's line and the thread's own border
both render

### 🎨 UI Changes

In RTL, the resize handle and thread panel border now appear as a single
clean line instead of two separated lines.

Author: oliverlaz

examples/vite/src/index.scss

@@ -122,8 +122,7 @@ body {
   .app-chat-resize-handle__hitbox {
     position: absolute;
     inset-block: 0;
-    inset-inline-start: 50%;
-    transform: translateX(-50%);
+    inset-inline-start: -6px;
     width: 12px;
     display: flex;
     justify-content: center;
@@ -438,6 +437,11 @@ body {
       max-width: none;
     }
 
+    /* Hide thread's own start border when resize handle provides the separator */
+    .app-chat-resize-handle--thread + .app-chat-thread-panel .str-chat__thread-container {
+      --str-chat__thread-border-inline-start: none;
+    }
+
     /* If space gets tight while sidebar + thread are both open,
        prioritize the thread and collapse the main panel. */
     @container (max-width: 1148px) {