Generate real glossy reel-symbol art and close the visual gap further

Hand-authored 6 gradient-shaded SVG icons (gold coin, purple gem,
green potion, blue RZ shield, coin stack, gift box) to replace the
flat monochrome placeholder icons in the reels and reward list - no
image-generation tool is available here, so this is genuine vector
art with radial/linear gradients and highlight layers rather than a
literal asset match to the mockup's renders.

Combined with real Godot techniques: a layered-label glow effect on
the "RZ" marquee, chase-lit marquee bulbs down both sides of the
reel screen (looping Tweens), a much more pronounced arch silhouette
and neon border on the cabinet, and an overall scale-up (bigger
reels, marquee, lever) so the machine reads as the dominant element
instead of a cramped placeholder. Verified with an actual rendered
screenshot (not just structural checks) at each iteration.

assets/icons/slots/slot_shield.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="64" height="64" viewBox="0 0 64 64">
+  <defs>
+    <linearGradient id="shieldBody" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#7fb4ff"/>
+      <stop offset="55%" stop-color="#2563eb"/>
+      <stop offset="100%" stop-color="#12377a"/>
+    </linearGradient>
+  </defs>
+  <path d="M32 4 52 12v16c0 16-9 26-20 32C21 54 12 44 12 28V12Z" fill="url(#shieldBody)" stroke="#0b1f4d" stroke-width="1.5"/>
+  <path d="M32 4 52 12v6L32 10 12 18v-6Z" fill="#ffffff" opacity="0.25"/>
+  <path d="M24 30h16v6l-8 10-8-10Z" fill="#ffffff" opacity="0.9"/>
+</svg>