fix(sidebar): check isMobile before collapsible=none; sticky desktop layout

- Swap isMobile and collapsible=none checks: mobile always gets Sheet
  overlay regardless of collapsible setting. Previously collapsible=none
  rendered an inline div on mobile, pushing content and preventing close.
- collapsible=none desktop path: use sticky top-0 h-svh instead of
  h-full so the sidebar stays visible while the page scrolls.
- Update sidebar-none-mode CSS to use align-items:flex-start which is
  required for sticky children in a flex container.

Root cause: stored sidebarCollapsible=none pref (set via theme
customizer) triggered this code path on both mobile and desktop.

src/components/ui/sidebar.tsx

@@ -172,24 +172,10 @@ function Sidebar({
 }) {
   const { isMobile, state, openMobile, setOpenMobile } = useSidebar()
 
-  if (collapsible === "none") {
-    return (
-      <div
-        data-slot="sidebar"
-        className={cn(
-          "bg-sidebar text-sidebar-foreground flex h-full w-(--sidebar-width) flex-col",
-          className
-        )}
-        {...props}
-      >
-        {children}
-      </div>
-    )
-  }
-
+  // Mobile check must come first — always use Sheet on mobile regardless of collapsible mode
   if (isMobile) {
     return (
-      <Sheet open={openMobile} onOpenChange={setOpenMobile} modal={false} {...props}>
+      <Sheet open={openMobile} onOpenChange={setOpenMobile} {...props}>
         <SheetContent
           data-sidebar="sidebar"
           data-slot="sidebar"
@@ -212,6 +198,21 @@ function Sidebar({
     )
   }
 
+  if (collapsible === "none") {
+    return (
+      <div
+        data-slot="sidebar"
+        className={cn(
+          "bg-sidebar text-sidebar-foreground sticky top-0 self-start flex h-svh w-(--sidebar-width) flex-col overflow-y-auto",
+          className
+        )}
+        {...props}
+      >
+        {children}
+      </div>
+    )
+  }
+
   return (
     <div
       className="group peer text-sidebar-foreground hidden md:block"