Render real skill + perk icons in the live UI

extract.py now also copies every referenced icon webp from sample-data into
character-builder/frontend/public/icons/ so Vite bundles them with the SPA
(serves at /icons/<filename>.webp). 154 icons (91 skill + 63 perk) end up
in the build, ~668 KB total.

SkillTree nodes show the skill's icon centered in the node instead of the
name text (name moves below the node as a label). Hovered / allocated /
maxed states change icon brightness and saturation, with a sand-colored
drop-shadow on maxed nodes.

XpProgressCard renders the perk icon to the left of each perk's text.
Locked perks desaturate the icon. The grid grows from 2 columns to 3 to
accommodate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

character-builder/data/index.json

@@ -47,5 +47,11 @@
       "nodes": 22,
       "edges": 22
     }
-  ]
+  ],
+  "icons": {
+    "directory": "frontend/public/icons",
+    "served_at": "/icons/",
+    "count": 154,
+    "missing": []
+  }
 }