Reorder research categories to make System the default first entry

- Set `defaultCategory` to "system" and move its definition ahead of "exploration" so the runtime-built system tech tree is presented first.

data/research.json

@@ -3,11 +3,11 @@
   "enabled": true,
   "timeUnit": "seconds",
   "maxConcurrent": 1,
-  "defaultCategory": "exploration",
+  "defaultCategory": "system",
   "categories": [
-    { "id": "exploration", "label": "Exploration", "accent": "#00e5ff" },
+    { "id": "system", "label": "System", "accent": "#5fd4ff", "dynamic": true },
     { "id": "mining", "label": "Mining", "accent": "#ffc94d" },
-    { "id": "system", "label": "System", "accent": "#5fd4ff", "dynamic": true }
+    { "id": "exploration", "label": "Exploration", "accent": "#00e5ff" }
   ],
   "_dynamic": "A category flagged `dynamic: true` has NO data/research/<id>.json — its tree is built at runtime per situation (js/research/SystemCategory.js: one tech pair per solar system, named after the system the player is in — its Map tech is granted on entry, its jumpgate tech researchable once every NAV point — planets, space stations, jump gates — is discovered, and activation flips the gates on, here and in the linked systems' return gates). Static categories keep the one-file-per-category rule; the tests exempt dynamic ones from the tree-file check.",
   "video": {