fix: scale thumbnails to fill the preview window

src/main.rs

@@ -9,6 +9,7 @@
 //! Esc closes the popup; j/k loads the selected video's description and i returns to editing.
 //! Normal-mode t toggles a thumbnail in the same-sized popup; j/k follows selection, Esc closes.
 //! Images use native terminal graphics when supported, otherwise colored half-block characters.
+//! Thumbnails scale up or down proportionally to fill the popup without cropping.
 //! curl fetches JPEGs lazily into memory; decoding and image resizing/encoding run off-thread.
 //! Channel categories live in `$XDG_CONFIG_HOME/tubular/config.toml`, or `~/.config/tubular/config.toml`.
 //! Under `[categories]`, write `Rust = ["@RustVideos"]`; channel URLs and UC IDs also work.