Remove bloat: the design bundle zip, duplicate images, course notebooks

- claude-design/Dolphin Beats Music Player.zip (32 MB): the design tool's export of
  the mockups already unpacked next to it. Still in history (30065bb) if needed.
- claude-design/pasted-*.png: byte-identical copies of dolphin-mascot.png and
  dolphin-remote.png that nothing references.
- python-backend/notebooks: the chord-recognition course material with stale paths.
  effect_debug.ipynb stays.

The hardware models are untouched, and the history keeps its size: removing files
from the tree does not shrink the pack.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/notebooks/README.md

@@ -1,14 +1,10 @@
 # Notebooks
 
-Exploratory material, **not part of the running backend** and not imported by it.
+Scratch work, **not part of the running backend** and not imported by it.
 
-`audio_analysis.py` and the three `C5S*` notebooks are chroma/chord-recognition course
-work (they still reference stale absolute paths). `effect_debug.ipynb` is scratch work
-for tinkering with LED effects.
-
-They need `librosa`, `numba` and `numpy`, which are deliberately not in the backend's
-dependencies:
+`effect_debug.ipynb` is for tinkering with LED effects. It needs `numpy` and `jupyter`,
+which are deliberately not in the backend's dependencies:
 
 ```sh
-pip install librosa numba numpy jupyter
+pip install numpy jupyter
 ```