fix(theme): at phone width the JS-off rail fallback is the measured worst case

booth-dev suggested this. At or below 480px, .item's scroll-margin
fallback is 205px, the 16-group rail measured at 390px. With JS on,
--rail-h is exact and nothing changes.

Measured on the same 76 jumps:
- JS off: 0 under the rail, previously 19. At 390px, where a short rail
  gets the full fallback, tiles overshoot by at most 74px, and they stay
  visible.
- JS on: unchanged, 0 under.

660 passed; visual order still matches document order on 32 renders.

booth/templates/base.html

@@ -622,6 +622,14 @@
   .bench-add input[type=url]{flex:2;min-width:0}
   .bench-add input[type=text]{flex:1;min-width:0}
 
+  /* JS-off fallback at phone width (booth-dev's suggestion, 2026-09-23): the
+     rail wraps tallest here, so the fallback is the MEASURED worst case —
+     205px, the 16-group live maximum at 390px. Overshooting a short rail
+     leaves the tile visible; undershooting hides it. JS on, --rail-h is exact
+     and this changes nothing. */
+  @media (max-width:480px){
+    .item{scroll-margin-top:calc(var(--rail-h, 205px) + 12px)}
+  }
   @media (max-width:600px){
     main{padding:20px 16px 40px}
     .topbar{padding:12px 16px}