Sea Select: spread center significator supplies the card-face image — TDD

The felt cross's CENTER significator was hardcoded to the corner-rank + suit-icon
text thumbnail, so an image deck (RWS / Minchiate) showed a bare "0"+icon card
instead of its face. Mirror my_sea: render the `.sig-stage-card-img` (image mode)
when the sig's deck has card images, else fall through to the text thumbnail. The
tray sig stays the simple thumbnail (user-spec). The sig's `deck_variant` is the
card's OWN deck — the Sig Select pick is drawn from the Role Select contributed
deck (`_room_deck_variant`), so this is the correct image source (no equipped_deck
bug, unlike the earlier FLIP back-img).

- IT: an image-deck significator renders the center `sea-sig-card sig-stage-card--image`
- 949+ epic ITs green

; FLIP tint tweak (parallel edit): flipped-back overlay alpha 0.6 → 0.3

[[project-deck-segment-model]]
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/static_src/scss/_card-deck.scss

@@ -2348,10 +2348,10 @@ $_sea-title-els: '.fan-card-name, .sig-qualifier-above, .sig-qualifier-below, .f
     }
 }
 .sea-stage--gravity .sea-stage-card.is-flipped-to-back {
-    &::after { background: rgba(var(--quiUser), 0.6); }
+    &::after { background: rgba(var(--quiUser), 0.3); }
 }
 .sea-stage--levity .sea-stage-card.is-flipped-to-back {
-    &::after { background: rgba(var(--terUser), 0.6); }
+    &::after { background: rgba(var(--terUser), 0.3); }
 }
 
 // Sea stat block — reuses sig-select stat-block sizing, scoped to sea-stage.