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).

Cargo.toml

@@ -14,6 +14,7 @@ members = [
     "crates/adapters/http-json",
     "crates/adapters/rss",
     "crates/adapters/media",
+    "crates/api-types",
     "crates/bootstrap",
     "crates/client-desktop",
 ]
@@ -36,6 +37,9 @@ config-sqlite = { path = "crates/adapters/config-sqlite" }
 http-api = { path = "crates/adapters/http-api" }
 axum = { version = "0.8", features = ["macros"] }
 tower-http = { version = "0.6", features = ["cors"] }
+api-types = { path = "crates/api-types" }
+thiserror = "2.0"
+anyhow = "1.0"
 serde = { version = "1.0", default-features = false, features = ["derive", "alloc"] }
 serde_json = "1.0"
 sqlx = { version = "0.8", features = ["runtime-tokio-rustls", "sqlite"] }