docs: add backend and frontend coding conventions
- docs/conventions/backend.md: architecture layers (Route→Service→Repository),
TypeBox + @fastify/type-provider-typebox as validation standard, naming,
error handling, config validation, DI via Fastify decorators
- docs/conventions/frontend.md: feature-based folder structure, Zustand state
rules, component/hook patterns, naming, TypeScript rules
- AGENTS.md: add Web Stack Conventions section referencing new docs,
update commit scopes for api, web, types, wasm
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
