feat: add backdrop blur overlay when mobile hamburger menu is open

ui/src/routes/+layout.svelte

@@ -745,14 +745,24 @@
 					</Button>
 				</div>
 
-				<!-- Click-outside overlay for dropdowns -->
-				{#if langMenuOpen || userMenuOpen}
-					<div
-						class="fixed inset-0 z-40"
-						onpointerdown={() => { langMenuOpen = false; userMenuOpen = false; }}
-						aria-hidden="true"
-					></div>
-				{/if}
+		<!-- Click-outside overlay for dropdowns -->
+			{#if langMenuOpen || userMenuOpen}
+				<div
+					class="fixed inset-0 z-40"
+					onpointerdown={() => { langMenuOpen = false; userMenuOpen = false; }}
+					aria-hidden="true"
+				></div>
+			{/if}
+
+			<!-- Backdrop for mobile hamburger menu -->
+			{#if menuOpen}
+				<div
+					class="fixed inset-0 z-40 sm:hidden"
+					style="background: rgba(0,0,0,0.6); backdrop-filter: blur(4px);"
+					onpointerdown={() => { menuOpen = false; }}
+					aria-hidden="true"
+				></div>
+			{/if}
 			{:else}
 				<div class="ml-auto">
 					<a