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/pyproject.toml

@@ -55,7 +55,7 @@ filterwarnings = ["error"]
 [tool.ruff]
 line-length = 100
 target-version = "py311"
-# Course material and scratch work, not part of the backend. See notebooks/README.md.
+# Scratch work, not part of the backend. See notebooks/README.md.
 extend-exclude = ["notebooks"]
 
 [tool.ruff.lint]