Add AppConfig loaded from .env via dotenvy. DATABASE_URL prefix
determines the backend (sqlite:// → SQLite, postgresql:// → PostgreSQL).
ConfigError via thiserror gives clear messages on missing or unknown URLs.
Server logs the chosen backend at startup.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Sets up the full project skeleton: Cargo.toml with ssr/hydrate features,
Axum server entry point, shared Leptos lib, root App component with router,
server/client module split, and Trunk config for WASM build.
Both `cargo check --features ssr` and `cargo check --features hydrate --target wasm32-unknown-unknown` pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
