test(library): read one "now" in the fixture and the page across midnight

The concept fixture recorded each prompt's last use with the database's wall
clock at seeding, while the page computed "today" from its own clock at
render. A run seeded at 23:59:5x UTC and rendered after midnight read "Used
yesterday" against the concept's "Used today" (PR #284, frontend coverage
shard 2/3).

The concept fixture now takes one instant (optional "now" through the
concept reset hook, the wall clock otherwise) for every last use, mtime and
run time it writes and returns it as manifest.now; seedConceptAt fixes the
browser's Date at that instant. The Library and Images concept specs seed
through it, and the two workspace specs that assert "Used today" and the
"Today" caption fix the page clock at the time they patch or seed. A new
test starts the browser at 23:59:58 UTC, seeds at that instant and passes
midnight before either document opens.

tests/browser/workspace-library.spec.mjs

@@ -610,7 +610,9 @@ test('card matches the concept and never wraps the category mid-word',async({pag
     await expect(await request.post('/prompt_manager/bulk/category',{data:{prompt_ids:[dual.id],category:''}})).toBeOK();
     // A long saved title fills the title row, so only the chip's own min-content keeps "Uncategorized" and "asian" whole.
     const wide='A remarkably long saved positive prompt title that fills the whole card title row';
-    await patchShelf(page,item=>item.id===cat.id?{run_count:4,last_used_at:new Date().toISOString(),rating:5}:item.id===dual.id||item.id===dog.id?{title:wide,run_count:1,last_used_at:null,rating:null}:{});
+    // One "now" for the patched last use and the page's clock, so it reads "Used today" on either side of midnight.
+    const usedAt=new Date();await page.clock.setFixedTime(usedAt);
+    await patchShelf(page,item=>item.id===cat.id?{run_count:4,last_used_at:usedAt.toISOString(),rating:5}:item.id===dual.id||item.id===dog.id?{title:wide,run_count:1,last_used_at:null,rating:null}:{});
     for(const [width,height] of [[1600,1100],[390,844]]) {
         await page.setViewportSize({width,height});await page.goto(route);await expect(cards(page)).toHaveCount(3);
         const card=page.locator(`article[data-prompt-id="${cat.id}"]`);