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/.gitignore

@@ -0,0 +1,5 @@
+node_modules/
+dist/
+*.local
+.vite/
+*.tsbuildinfo