refactor: merge workspace (core/ + ui/) into a single flat binary crate

integrate CLI into main.rs, no lib.rs by design (YAGNI)

src/render/mod.rs

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/render/mod.rs
+//
+// Rendering subsystem: converts document content into displayable output.
+
+pub mod converter;
+pub mod thumbnail;