feat: warn before leaving a form with unsaved changes

Drag-dropping page images then clicking a nav link silently discarded the
whole form. Add a reusable guardUnsavedChanges helper (SvelteKit beforeNavigate
confirm + native beforeunload) and wire it into the manga upload, manga edit,
and chapter upload forms. The dirty check returns false while submitting so the
post-save redirect isn't prompted.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/unsaved-guard.ts

@@ -0,0 +1,36 @@
+import { beforeNavigate } from '$app/navigation';
+import { onMount } from 'svelte';
+
+/**
+ * Warn before leaving a page with unsaved form state. Call once from a
+ * component's script (during init). `isDirty` is read live on each navigation,
+ * so pass a closure over the form's reactive state — and have it return `false`
+ * once a save has succeeded (or is in flight) so the post-save redirect doesn't
+ * prompt.
+ *
+ * Covers both in-app navigation (SvelteKit `beforeNavigate`, with a confirm) and
+ * full-page unload / tab close / reload (the native `beforeunload` prompt).
+ */
+export function guardUnsavedChanges(isDirty: () => boolean): void {
+    beforeNavigate((nav) => {
+        // `leave` (tab close / reload) can't show a custom confirm here — the
+        // beforeunload handler below covers it. Only guard in-app navigations.
+        if (nav.type === 'leave') return;
+        if (!isDirty()) return;
+        if (!confirm('You have unsaved changes. Leave this page and discard them?')) {
+            nav.cancel();
+        }
+    });
+
+    onMount(() => {
+        const handler = (e: BeforeUnloadEvent) => {
+            if (isDirty()) {
+                e.preventDefault();
+                // Legacy browsers require returnValue to be set.
+                e.returnValue = '';
+            }
+        };
+        window.addEventListener('beforeunload', handler);
+        return () => window.removeEventListener('beforeunload', handler);
+    });
+}