feat: initial scaffold of uniweb — unibus web frontend (SPA + gateway)

Extracted from unibus v0.13.0: the chat SPA (web/, React+Mantine, per-user
BIP39 wallet) and the web gateway (cmd/webgw, REST+SSE) that acts as a bus
peer for the browser. Consumes unibus as a Go module via replace => ../unibus,
keeping its own replace fn-registry for the cybersecurity primitives.

go build/vet/test and pnpm build green in the new location.

web/postcss.config.cjs

@@ -0,0 +1,14 @@
+module.exports = {
+  plugins: {
+    "postcss-preset-mantine": {},
+    "postcss-simple-vars": {
+      variables: {
+        "mantine-breakpoint-xs": "36em",
+        "mantine-breakpoint-sm": "48em",
+        "mantine-breakpoint-md": "62em",
+        "mantine-breakpoint-lg": "75em",
+        "mantine-breakpoint-xl": "88em",
+      },
+    },
+  },
+};