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_concept_fixture.py

@@ -11,6 +11,7 @@ the same sidebar counts (24 images, 6 prompts, 15 tags). Nothing is mocked.
 from __future__ import annotations
 
 from dataclasses import dataclass
+import datetime
 import json
 import math
 import os
@@ -157,6 +158,8 @@ CONCEPT_MODEL = "illustrative-model.safetensors"
 MAX_EXTRA_IMAGES = 120
 # Unlinked, seedless images (D11 "W × H · PNG") newer than the concept's, for D3 checks.
 MAX_SEEDLESS_IMAGES = 2
+# A test's chosen "now" (epoch seconds) stays before the year 3000.
+MAX_EPOCH_SECONDS = 32_503_680_000
 SEEDLESS_SIZE = (1536, 1152)
 # The same 4:3 shape with a short "96 × 72 · PNG" subtitle that stays on one line at
 # 390px beside the card's Info and Tag (the D3 footer check).
@@ -342,6 +345,17 @@ def _bounded(name: str, value: int, maximum: int) -> None:
         raise ValueError(f"{name} is out of range")
 
 
+def _instant(now: float | None) -> float:
+    """The fixture's one "now": ``now`` itself, or the wall clock when omitted."""
+    if now is None:
+        return time.time()
+    if isinstance(now, bool) or not isinstance(now, (int, float)):
+        raise TypeError("now must be a number of epoch seconds")
+    if not 0 < now < MAX_EPOCH_SECONDS:
+        raise ValueError("now is out of range")
+    return float(now)
+
+
 def seed_workspace_concept(
     api: PromptManagerAPI,
     root: Path,
@@ -349,9 +363,16 @@ def seed_workspace_concept(
     extra_images: int = 0,
     seedless_images: int = 0,
     short_seedless: bool = False,
+    now: float | None = None,
 ) -> dict:
     """Replace this server's disposable rows and files with the concept's data.
 
+    Every relative time the fixture writes (the prompts' last use, the files'
+    mtimes, the runs' observation times) is taken from one instant, ``now``
+    (epoch seconds, the wall clock when omitted), returned as ``manifest["now"]``.
+    A browser whose clock is fixed at that instant reads the concept's "Used
+    today" and caption day however close to midnight the run starts.
+
     ``extra_images`` continues the concept's rounds (same prompts, dimensions and
     seed rule) after its 24 images, for bounded-rendering and Load more checks.
     ``seedless_images`` adds unlinked PNGs without metadata, newest of all;
@@ -361,6 +382,10 @@ def seed_workspace_concept(
         raise TypeError("short_seedless must be a boolean")
     _bounded("extra_images", extra_images, MAX_EXTRA_IMAGES)
     _bounded("seedless_images", seedless_images, MAX_SEEDLESS_IMAGES)
+    now = _instant(now)
+    used_at = datetime.datetime.fromtimestamp(now, datetime.timezone.utc).isoformat(
+        timespec="microseconds"
+    )
     _clear(api, root)
     db = api.db
     epoch = db.get_execution_epoch()
@@ -385,8 +410,15 @@ def seed_workspace_concept(
         db.update_prompt_pair_metadata(pair_ids[0], title=first_name)
         for name, text in prompt.negatives[1:]:
             pair_ids.append(db.save_prompt_pair(prompt.positive, text, title=name))
-        # Real queue-hook counting (D23): four runs, the last one now.
+        # Real queue-hook counting (D23): four runs, the last one now, at the
+        # fixture's own instant rather than the database's later wall clock.
         db.record_prompt_use(positive_id, times=CONCEPT_RUN_COUNT)
+        with db.model.get_connection() as conn:
+            conn.execute(
+                "UPDATE prompts SET last_used_at = ? WHERE id = ?",
+                (used_at, positive_id),
+            )
+            conn.commit()
         prompts.insert(
             0, {"id": prompt.id, "positive_id": positive_id, "pair_ids": pair_ids}
         )
@@ -400,7 +432,6 @@ def seed_workspace_concept(
     favorite_root = _favorite_root_key(list(api._get_all_output_dirs())[0])
     total = len(CONCEPT_PROMPTS) * CONCEPT_IMAGE_ROUNDS + extra_images
     rounds = math.ceil(total / len(CONCEPT_PROMPTS))
-    now = time.time()
     images, runs = [], []
     slots = [
         (round_index, prompt_index, prompt)
@@ -484,6 +515,8 @@ def seed_workspace_concept(
     api.invalidate_gallery_cache()
     return {
         "epoch": epoch,
+        # The fixture's "now" (epoch seconds), for the browser's fixed clock.
+        "now": now,
         # The newest file's mtime (epoch seconds), for the D17 caption date.
         "newest_modified": now - 10 if seedless_images else now - 60,
         "seedless": seedless,