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

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

core/src/render/mod.rs

@@ -1,7 +0,0 @@
-// 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;