fix(fileloader): revoke blob URLs before re-assignment and on destroy

Calling loadFile a second time previously leaked the previous object URL.
Add URL.revokeObjectURL(fileUrl) before creating a new one and in
onDestroy so all URLs are freed. Revoke behavior will be covered by the
useFileLoader hook tests in the next commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { onMount } from 'svelte';
+import { onMount, onDestroy } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import DocumentTopBar from '$lib/components/DocumentTopBar.svelte';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
@@ -31,6 +31,7 @@ $effect(() => {
 async function loadFile(id: string) {
 	isLoading = true;
 	fileError = '';
+	if (fileUrl) URL.revokeObjectURL(fileUrl);
 	fileUrl = '';
 
 	try {
@@ -51,6 +52,10 @@ async function loadFile(id: string) {
 	}
 }
 
+onDestroy(() => {
+	if (fileUrl) URL.revokeObjectURL(fileUrl);
+});
+
 // ── Mode state ───────────────────────────────────────────────────────────────
 
 let transcribeMode = $state(false);