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>

docs/REPO_OVERVIEW.md

@@ -116,8 +116,7 @@ reported at once.
   on numpy's stubs for two weeks without anyone noticing) is a failure, not a skip.
 - Prefer adding a scenario in `python-backend/scenarios/` over a hand-rolled test when
   the behaviour is end-to-end — those files are executed by the test suite.
-- `python-backend/notebooks/` is university course material on chord recognition, not
-  part of the app.
+- `python-backend/notebooks/` is LED-effect scratch work, not part of the app.
 - The firmware has no automated tests beyond a PlatformIO `native` env for LED effects.
 - The front-end has `vitest` over the pure modules (`web/src/lib/`) and `tsc --noEmit`;
   there is no component-level test harness. `npm run test` and `npx tsc --noEmit` are