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/adapters/tcp-server/src/error.rs

@@ -1,14 +1,7 @@
-#[derive(Debug)]
+#[derive(Debug, thiserror::Error)]
 pub enum TcpServerError {
-    Io(std::io::Error),
-    Encode(postcard::Error),
-}
-
-impl std::fmt::Display for TcpServerError {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            TcpServerError::Io(e) => write!(f, "io: {e}"),
-            TcpServerError::Encode(e) => write!(f, "encode: {e}"),
-        }
-    }
+    #[error("io: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("encode: {0}")]
+    Encode(#[from] postcard::Error),
 }