fix: Accept time:0 entries and precompute history entry URLs

Code-review follow-ups:
- validateEntry no longer rejects entries whose time is 0 (epoch); it now
  checks `typeof time === 'number'`, so restoring/uploading such entries
  works instead of silently dropping them.
- History.svelte serializes each entry's "open in new tab" URL once via a
  $derived list instead of calling serializeState (pako deflate) per row on
  every render.

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

src/lib/components/History/History.svelte

@@ -105,6 +105,11 @@
   // Absolute editor URL for an entry, so the link can be opened in a new tab or copied.
   const entryUrl = (state: State): string =>
     `${window.location.origin}${window.location.pathname}#${serializeState(state)}`;
+
+  // Serialize each entry's URL once per change rather than per row on every render.
+  const entriesWithUrl = $derived(
+    historyState.entries.map((entry) => ({ ...entry, openUrl: entryUrl(entry.state) }))
+  );
 </script>
 
 <Card onselect={tabSelectHandler} isOpen isClosable={false} {tabs} activeTabID={historyState.mode}>
@@ -143,8 +148,8 @@
     </div>
   {/snippet}
   <ul class="flex h-full min-w-fit flex-col gap-2 overflow-auto p-2" id="historyList">
-    {#if historyState.entries.length > 0}
-      {#each historyState.entries as { id, state, time, name, url, type } (id)}
+    {#if entriesWithUrl.length > 0}
+      {#each entriesWithUrl as { id, state, time, name, url, type, openUrl } (id)}
         <li class="flex flex-col gap-2">
           <div class="flex items-center justify-between">
             <div class="flex flex-col">
@@ -167,7 +172,7 @@
                 {dayjs(time).fromNow()}
               </span>
               <Button
-                href={entryUrl(state)}
+                href={openUrl}
                 target="_blank"
                 rel="noopener"
                 size="icon"

src/lib/components/History/historyState.svelte.ts

@@ -158,7 +158,7 @@ export const clearActive = (): void => {
 };
 
 const validateEntry = (entry: HistoryEntry): boolean =>
-  Boolean(entry && entry.type && entry.state && entry.time);
+  Boolean(entry && entry.type && entry.state) && typeof entry.time === 'number';
 
 export interface RestoreResult {
   restored: number;

src/lib/components/History/historyState.test.ts

@@ -241,6 +241,15 @@ describe('restoreEntries', () => {
     ]);
     expect(entriesFor('manual').map((e) => e.time)).toEqual([30, 20, 10]);
   });
+
+  it('restores entries whose time is 0 (epoch) instead of treating them as invalid', () => {
+    const result = restoreEntries([
+      { id: 'm0', name: 'epoch', state: defaultState, time: 0, type: 'manual' }
+    ]);
+    expect(result.restored).toBe(1);
+    expect(result.invalid).toBe(0);
+    expect(entriesFor('manual')).toHaveLength(1);
+  });
 });
 
 describe('injectHistoryIDs migration', () => {