extract api-types crate, adopt thiserror for all errors

api-types: standalone crate with DTOs (widget, data source, layout, preset)
extracted from http-api. Shared between http-api and future SPA.

thiserror: replaced all manual Display impls with derive macros across
8 crates (config-sqlite, config-memory, tcp-server, tcp-client,
http-json, rss, media, application).

crates/api-types/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "api-types"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+domain.workspace = true
+serde.workspace = true