feat: add minimal Gradio web UI (app.py)

Single-page form wrapping src.generate.generate_period: pick model, mode,
key, style, function, time, sampling params and optional prefix; returns
the chord grid plus downloadable .chord and .mid files. Russian usage
instructions are embedded on the same page.

Auto-length output is capped at 16 bars (the period maximum) so a model
that never emits EOS can't run away into dozens of NC/hold bars.

Added per the author's explicit request — web UI was previously out of
scope; updated CLAUDE.md and README accordingly. Choices for style/
function/time are derived from VOCAB so the form can't drift from the
tokenizer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -158,7 +158,7 @@ When generating commit messages or code comments, write in English. When generat
 - **Do not change the `.chord` format** without first updating `docs/chord_format_spec.md` and bumping its version number. The format is the contract between the human-readable data and the model; changing one side silently breaks everything.
 - **Do not modify files in `data/holdout/` or use them during training.** Holdout is held out.
 - **Do not add new model architectures "to compare"** unless explicitly asked. One model, done well, beats four half-done.
-- **Do not implement bells and whistles** (web UI, real-time audio synthesis, beam search, voicing models). They are explicitly out of scope.
+- **Do not implement bells and whistles** (real-time audio synthesis, beam search, voicing models). They are explicitly out of scope. (Exception: a minimal Gradio web UI in `app.py` was added at the author's explicit request — maintain it, but do not expand it into a full editor/DAW.)
 - **Do not silently round or coerce unrecognized chord symbols.** If a chord can't be parsed, raise an error with the file name, bar number, and position. Silent corruption of training data is the worst failure mode here.
 
 ## Things to always do
@@ -175,7 +175,7 @@ When generating commit messages or code comments, write in English. When generat
 - Voice leading / voicing inside chords above the bass
 - Rhythmic patterns inside a held chord
 - Arrangement, timbre, dynamics
-- Web interface / GUI
+- Web interface / GUI beyond the minimal Gradio form in `app.py` (added by explicit request; no notation editor, playback, or version history)
 - Real-time MIDI integration with REAPER
 - Modulation handling inside a single period
 - J-Pop fine-tuning experiment (future work after coursework deadline)