feat: refactor PDF extraction to caption-based screenshots, add upvote refresh, clean up UI

- PDF extractor: rewrite from embedded bitmap extraction to caption-based
  page region screenshots. Finds Figure/Table captions via regex,截取上方/下方
  page region, handles compound figures and vector graphics.
- Upvote refresh: new crawler.refresh_upvotes() re-fetches upvotes for recent
  N days without inserting new papers. Scheduler runs daily 30min after pipeline.
- Admin: add /admin/refresh-upvotes endpoint and dashboard button.
- UI: remove date quick nav, show upvote update time on detail/card pages,
  clean up CSS date-chip styles.
- Utils: add recent_date_strs() helper.

app/static/css/style.css

@@ -138,20 +138,7 @@ a:hover {
 }
 
 /* ── Date Quick Nav ─────────────────────────────────────────────── */
-.date-quick-nav {
-  margin-top: 32px;
-  padding-top: 16px;
-  border-top: 1px solid var(--border);
-  font-size: 0.85rem;
-  color: var(--ink-light);
-  display: flex;
-  align-items: center;
-  gap: 8px;
-  flex-wrap: wrap;
-}
-
 /* ── Chips (shared) ─────────────────────────────────────────────── */
-.date-chip,
 .tag-chip,
 .filter-chip {
   display: inline-block;
@@ -162,14 +149,12 @@ a:hover {
   font-size: 0.8rem;
   color: var(--ink-light);
 }
-.date-chip:hover,
 .tag-chip:hover,
 .filter-chip:hover {
   border-color: var(--accent);
   color: var(--accent);
   text-decoration: none;
 }
-.date-chip.active,
 .tag-chip.active,
 .filter-chip.active {
   background: var(--accent);
@@ -352,6 +337,11 @@ a:hover {
   margin-bottom: 12px;
 }
 
+.detail-upvote-time {
+  font-size: 0.78rem;
+  color: var(--border);
+}
+
 .detail-tags {
   margin-bottom: 12px;
   display: flex;