Roll out refined-minimal styling and product thumbnails across pages

Apply the dashboard's refined direction to the rest of the app:
- Items: page-title header, product thumbnail (best-price image) beside
  each item name via the shared thumbSm component.
- Results: refined headers; per-item and global tables get standardized
  thumbnails (global table gains a thumbnail column).
- Settings: page-title + subtitle, section headings restyled.
- Item form: page-title header.
- Shared: thumbSm component + compact thumbnail CSS variant.

static/css/app.css

@@ -171,6 +171,9 @@ a:hover { text-decoration: underline; }
   font-weight: 600;
   text-transform: uppercase;
 }
+/* Compact variant for table rows. */
+.v-thumb-sm { width: 40px; height: 40px; border-radius: 7px; }
+.v-thumb-sm.v-thumb-empty { font-size: 0.9rem; }
 
 .v-feed { display: block; }
 .v-feed-row {