Tighten item icon viewBoxes so art fills the slot

Each icon's viewBox is set to a square box around its actual content
(with a stroke-width margin + small breathing room), leaving the path
data unchanged. Under-filled icons (ingots, plates, silicon, etc.) now
scale up and recenter to fill the half-tile item slot; already-full
icons are left as-is (clamped to only ever zoom in). Fixes items looking
too small in the game world.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/icons/items/iron_ingot.svg

@@ -1,4 +1,4 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="24.0 28.0 62.0 62.0" width="100" height="100">
   <g stroke="#2f343b" stroke-width="3.5" stroke-linejoin="round">
     <path d="M28 72 L72 72 L64 54 L36 54 Z" fill="#a2a9b0"/>
     <path d="M64 54 L72 72 L82 64 L74 46 Z" fill="#868d95"/>