feat(web): mobileNav store for off-canvas drawer state

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/stores/mobileNav.svelte.ts

@@ -0,0 +1,22 @@
+// Open/close state for the off-canvas mobile navigation drawer.
+// Mounted once from Shell.svelte; toggled by the hamburger button.
+
+let _open = $state(false);
+
+export const mobileNav = {
+  get value(): boolean {
+    return _open;
+  }
+};
+
+export function openMobileNav(): void {
+  _open = true;
+}
+
+export function closeMobileNav(): void {
+  _open = false;
+}
+
+export function toggleMobileNav(): void {
+  _open = !_open;
+}