feat: add Shift+Tab button to mobile keyboard accessory bar

Mobile users cannot press Shift+Tab on virtual keyboards. Add a ⇧Tab
button that sends the escape sequence (\x1b[Z) to the PTY, enabling
mode switching on mobile devices.

Also fix accessory bar overflow on narrow screens by making it
horizontally scrollable with hidden scrollbar.

src/web/public/mobile.css

@@ -886,7 +886,8 @@ html.mobile-init .file-browser-panel {
     padding-right: calc(8px + var(--safe-area-right));
     gap: 8px;
     align-items: center;
-    justify-content: center;
+    overflow-x: auto;
+    -webkit-overflow-scrolling: touch;
     z-index: 51;
     transition: transform 0.15s ease-out;
     will-change: transform;
@@ -896,10 +897,16 @@ html.mobile-init .file-browser-panel {
     display: flex;
   }
 
+  /* 隐藏滚动条但保留滑动能力 */
+  .keyboard-accessory-bar::-webkit-scrollbar {
+    display: none;
+  }
+
   .accessory-btn {
     display: inline-flex;
     align-items: center;
     justify-content: center;
+    flex-shrink: 0;
     gap: 4px;
     padding: 6px 12px;
     background: #2a2a2a;
@@ -2068,7 +2075,8 @@ html.mobile-init .file-browser-panel {
   padding: 6px 8px;
   gap: 8px;
   align-items: center;
-  justify-content: center;
+  overflow-x: auto;
+  -webkit-overflow-scrolling: touch;
   z-index: 51;
 }
 
@@ -2076,10 +2084,15 @@ html.mobile-init .file-browser-panel {
   display: flex;
 }
 
+.keyboard-accessory-bar::-webkit-scrollbar {
+  display: none;
+}
+
 .accessory-btn {
   display: inline-flex;
   align-items: center;
   justify-content: center;
+  flex-shrink: 0;
   gap: 4px;
   padding: 6px 12px;
   background: #2a2a2a;