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/routes/manga/[id]/edit/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError, fileUrl } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import {
         deleteMangaCover,
         updateManga,
@@ -49,6 +50,25 @@
         mangaTitle.trim().length > 0 && !coverError && !submitting
     );
 
+    // Warn before leaving with unsaved edits. `submitting` stays true through
+    // the post-save redirect, so that navigation isn't prompted.
+    guardUnsavedChanges(
+        () =>
+            !submitting &&
+            (mangaTitle !== data.manga.title ||
+                mangaStatus !== data.manga.status ||
+                mangaDescription !== (data.manga.description ?? '') ||
+                JSON.stringify(mangaAuthors) !==
+                    JSON.stringify(data.manga.authors.map((a) => a.name)) ||
+                JSON.stringify(mangaAltTitles) !== JSON.stringify([...data.manga.alt_titles]) ||
+                JSON.stringify(mangaGenreIds) !==
+                    JSON.stringify(data.manga.genres.map((g) => g.id)) ||
+                coverFile != null ||
+                pendingCoverRemoval ||
+                authorDraft.trim() !== '' ||
+                altTitleDraft.trim() !== '')
+    );
+
     function addAuthor() {
         const name = authorDraft.trim();
         if (!name) return;

frontend/src/routes/manga/[id]/upload-chapter/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError, fileUrl } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import { createChapter } from '$lib/api/chapters';
     import { session } from '$lib/session.svelte';
     import ChapterPagesEditor, {
@@ -19,6 +20,16 @@
     let submitting = $state(false);
     let error: string | null = $state(null);
 
+    // Warn before leaving with staged pages / edits. `submitting` stays true
+    // through the post-save redirect, so that navigation isn't prompted.
+    guardUnsavedChanges(
+        () =>
+            !submitting &&
+            (pages.length > 0 ||
+                title.trim() !== '' ||
+                (number != null && number !== data.defaultNumber))
+    );
+
     const allPagesValid = $derived(pages.every((p) => !p.error));
     const canSubmit = $derived(
         Boolean(session.user) &&

frontend/src/routes/upload/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import { createManga, type MangaStatus } from '$lib/api/mangas';
     import { createChapter } from '$lib/api/chapters';
     import { session } from '$lib/session.svelte';
@@ -43,6 +44,23 @@
     let mangaError = $state<string | null>(null);
     let success = $state<string | null>(null);
 
+    // Warn before navigating away with an in-progress upload. Suppressed while
+    // submitting and after a successful save (which redirects).
+    guardUnsavedChanges(
+        () =>
+            !success &&
+            !submitting &&
+            (mangaTitle.trim() !== '' ||
+                mangaDescription.trim() !== '' ||
+                mangaAuthors.length > 0 ||
+                mangaAltTitles.length > 0 ||
+                mangaGenreIds.length > 0 ||
+                coverFile != null ||
+                stagedChapters.length > 0 ||
+                authorDraft.trim() !== '' ||
+                altTitleDraft.trim() !== '')
+    );
+
     const allChapterPagesValid = $derived(
         stagedChapters.every((c) => c.pages.every((p) => !p.error))
     );