feat(ui:delta upload): warn user if refresh page during upload

frontend/src/routes/+layout.svelte

@@ -15,6 +15,8 @@
 	import { hashUrlToPath } from '$lib/utils/hashRedirect';
 	import { killLegacyServiceWorker } from '$lib/utils/killLegacyServiceWorker';
 	import { getOidcProviders, type OidcProviders } from '$lib/api/endpoints/auth';
+	import { readAndClearInterrupted } from '$lib/upload/interruption';
+	import { t } from '$lib/i18n/index.svelte';
 
 	let { children } = $props();
 
@@ -156,6 +158,32 @@
 		// loading state) is already in the DOM behind it.
 		document.getElementById('app-splash')?.remove();
 
+		// If a page reload interrupted one or more uploads mid-flight, the
+		// registry in sessionStorage carries breadcrumbs into the next mount.
+		// Toast a resume hint — already-uploaded chunks reuse via delta's
+		// negotiate stage, so re-dropping the same file is fast, not from
+		// scratch. Reading the register clears it so a subsequent reload
+		// doesn't re-notify.
+		const interrupted = readAndClearInterrupted();
+		if (interrupted.length > 0) {
+			const one = interrupted.length === 1;
+			const msg = one
+				? t(
+						'files.upload_interrupted_one',
+						{ description: interrupted[0].description },
+						`Upload interrupted: ${interrupted[0].description}. Re-drop to resume — already-uploaded chunks are reused.`
+					)
+				: t(
+						'files.upload_interrupted_many',
+						{ n: interrupted.length },
+						`${interrupted.length} uploads interrupted. Re-drop the files to resume — already-uploaded chunks are reused.`
+					);
+			// 10 s dwell — same pattern as the other long-copy toasts
+			// (see profile SSO error). Info kind because the situation is
+			// recoverable and the user only needs to know how to resume.
+			ui.notify(msg, 'info', 10000);
+		}
+
 		// Redirect old `#/...` bookmarks to the new path before anything else.
 		if (typeof location !== 'undefined' && location.hash.startsWith('#/')) {
 			// hashUrlToPath returns a dynamic in-app path string; resolve() is typed

frontend/src/routes/files/[...path]/+page.svelte

@@ -30,6 +30,12 @@
 		resolveOwnedHashes,
 		tryDeltaUpload
 	} from '$lib/api/endpoints/deltaUpload';
+	import {
+		acquireUploadGuard,
+		markUploadFinished,
+		markUploadStarted,
+		releaseUploadGuard
+	} from '$lib/upload/interruption';
 	import { addFavorite, removeFavorite } from '$lib/api/endpoints/favorites';
 	import { canEditWithWopi, getEditorUrlWithFallback } from '$lib/api/endpoints/wopi';
 	import { addTracks, createPlaylist, listPlaylists } from '$lib/api/endpoints/music';
@@ -710,6 +716,17 @@
 	async function uploadBatch(files: File[]) {
 		if (files.length === 0) return;
 		uploading = true;
+		// Arm the reload-guard + persist a "batch in flight" marker so a
+		// page refresh mid-upload (a) prompts the browser's "Leave site?"
+		// dialog and (b) leaves a breadcrumb the layout picks up on the
+		// next mount → toasts a "uploads were interrupted, re-drop to
+		// resume (chunks reuse)" hint.
+		acquireUploadGuard();
+		const batchDescription =
+			files.length === 1
+				? files[0].name
+				: t('files.n_files_batch', { n: files.length }, `${files.length} files`);
+		const batchHandle = markUploadStarted(batchDescription, currentId);
 		const nid = ui.startProgress(
 			t('files.uploading_n', { done: 0, total: files.length }, `Uploading 0/${files.length} files…`)
 		);
@@ -760,6 +777,8 @@
 			ui.finishProgress(nid, errorMessage(err), 'error');
 		} finally {
 			uploading = false;
+			markUploadFinished(batchHandle);
+			releaseUploadGuard();
 		}
 	}
 
@@ -1539,6 +1558,14 @@
 	async function uploadTree(entries: { file: File; relativePath: string }[]) {
 		if (entries.length === 0) return;
 		uploading = true;
+		// Same reload-guard + interrupted-uploads breadcrumb as uploadBatch —
+		// the browser prompts on refresh, and if the user reloads anyway
+		// the layout picks up the marker on next mount and toasts.
+		acquireUploadGuard();
+		const treeHandle = markUploadStarted(
+			t('files.n_files_batch', { n: entries.length }, `${entries.length} files`),
+			currentId
+		);
 		// Same bell progress notification as uploadBatch, so folder uploads show
 		// live progress + a final result instead of staying silent until the end.
 		const nid = ui.startProgress(
@@ -1594,6 +1621,8 @@
 			ui.finishProgress(nid, errorMessage(err), 'error');
 		} finally {
 			uploading = false;
+			markUploadFinished(treeHandle);
+			releaseUploadGuard();
 		}
 	}