feat(ui): improve mobile search and consolidate language selector

- On mobile (≤768px), replace the full search bar in the topbar with a
  search icon button; tapping it expands a full-width search overlay
  with a back arrow to collapse (Google Drive-style pattern)
- Escape key also collapses the mobile search overlay
- Hide the redundant inline search icon when the mobile search bar is
  active (submit button already has one)
- Move the language selector from the topbar into the user menu for all
  screen sizes, reducing topbar clutter; it renders as an accordion row
  matching other menu items
- Fix user menu positioning in RTL layouts (Arabic/Persian): anchor to
  left:0 instead of right:0 so the panel opens inward rather than
  off-screen

static/css/components/userMenu.css

@@ -48,6 +48,7 @@
     position: absolute;
     top: calc(100% + 10px);
     right: 0;
+    left: auto;
     width: 300px;
     background: var(--color-bg-surface);
     border-radius: 16px;
@@ -63,6 +64,11 @@
     display: block;
 }
 
+[dir="rtl"] .user-menu {
+    right: auto;
+    left: 0;
+}
+
 @keyframes userMenuIn {
     from {
         opacity: 0;