Prepare product layout and reusable Flutter theme

Relocate the existing Astro website and preserve its assets and hosting configuration. Add the consumer/backend scaffold and themed Flutter package with consumer tests. Related to #1.

website/design/tokens.json

@@ -0,0 +1,24 @@
+{
+  "name": "SXCHNGE",
+  "version": 1,
+  "colors": {
+    "ink": "#14221d",
+    "mint": "#c4f9ce",
+    "lime": "#d7ff5f",
+    "paper": "#f9faf6",
+    "muted": "#637168",
+    "line": "#d9e0d8",
+    "forest": "#396a4b",
+    "focus": "#168850",
+    "darkField": "#25372e",
+    "darkReceive": "#203d2d",
+    "sageSurface": "#eaf0e6",
+    "blueSurface": "#edf0f9"
+  },
+  "typography": { "family": "Onest", "bodySize": 16, "bodyLineHeight": 1.65 },
+  "website": {
+    "radius": { "card": 24, "input": 14, "pill": 999 },
+    "spacing": [4, 8, 12, 16, 24, 32, 48, 64, 96],
+    "motionMs": 200
+  }
+}