Sync from GitHub via hub-sync

[NAH-35] make the recap reachable: Finish & Keep studying buttons (#16)

[NAH-34] fix Gradio 6 theme placement + pin gradio version (#15)

[NAH-32] difficulty toggle UI: make-it-harder / make-it-easier (#14)

[NAH-31] visual polish: card X of N progress, clearer cues (#13)

[NAH-30] loading spinners on every model call + button locking (#12)

[NAH-23] handle bad inputs: empty / image-only / corrupt PDF -> clear error (#11)

[NZH-17] extract_text (PDF + pasted/.txt) + length-based chunker

Add app, content pipeline, learning engine, and gitignore