feat: feed UI, in-app player, offline mode, settings

Adds an end-to-end integration test that runs the real pipeline against
live YouTube Atom feeds, verifying the merged feed is newest-first
across channels.

src-tauri/src/db.rs

@@ -62,6 +62,12 @@ impl Db {
         Self::init(conn)
     }
 
+    /// In-memory database, for tests only.
+    pub fn open_in_memory_pub() -> Result<Db, String> {
+        let conn = Connection::open_in_memory().map_err(|e| e.to_string())?;
+        Self::init(conn)
+    }
+
     #[cfg(test)]
     pub fn open_in_memory() -> Result<Db, String> {
         let conn = Connection::open_in_memory().map_err(|e| e.to_string())?;