guard around potential crashes

src/lib/streamCache.svelte.ts

@@ -1,5 +1,6 @@
 import { browser } from '$app/environment';
 import { SvelteSet, SvelteMap } from 'svelte/reactivity';
+import { readStored, writeStored } from './utils.ts';
 import type { StreamSummary, PartialDownload } from './types.ts';
 
 const STORAGE_KEY = 'cachedStreams';
@@ -16,23 +17,21 @@ const CHUNK_SIZE = 8 * 1024 * 1024;
 // as the intent is to catch socket breakage during cellular network changes for example
 const CHUNK_TIMEOUT = 60_000;
 
-export const cached = new SvelteSet<string>(
-    browser ? JSON.parse(localStorage.getItem(STORAGE_KEY) || '[]') : []
-);
+export const cached = new SvelteSet<string>(readStored<string[]>(STORAGE_KEY, []));
 export const downloading = new SvelteMap<string, number>();
 
 // bytes already written to OPFS for each incomplete download
 export const partials = new SvelteMap<string, PartialDownload>(
-    browser ? Object.entries(JSON.parse(localStorage.getItem(PARTIALS_KEY) || '{}')) : []
+    Object.entries(readStored<Record<string, PartialDownload>>(PARTIALS_KEY, {}))
 );
 
 if (browser) {
     $effect.root(() => {
         $effect(() => {
-            localStorage.setItem(STORAGE_KEY, JSON.stringify([...cached]));
+            writeStored(STORAGE_KEY, [...cached]);
         });
         $effect(() => {
-            localStorage.setItem(PARTIALS_KEY, JSON.stringify(Object.fromEntries(partials)));
+            writeStored(PARTIALS_KEY, Object.fromEntries(partials));
         });
     });
 }