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-client/src/lib.rs

@@ -4,23 +4,16 @@ use std::time::Duration;
 use client_domain::NetworkPort;
 use protocol::MAX_FRAME_SIZE;
 
-#[derive(Debug)]
+#[derive(Debug, thiserror::Error)]
 pub enum TcpClientError {
-    Io(std::io::Error),
+    #[error("io: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("not connected")]
     NotConnected,
+    #[error("frame too large: {0}")]
     FrameTooLarge(usize),
 }
 
-impl std::fmt::Display for TcpClientError {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            TcpClientError::Io(e) => write!(f, "io: {e}"),
-            TcpClientError::NotConnected => write!(f, "not connected"),
-            TcpClientError::FrameTooLarge(n) => write!(f, "frame too large: {n}"),
-        }
-    }
-}
-
 pub struct StdTcpClient {
     stream: Option<TcpStream>,
 }