docs+chore: add screenshot to README; drop committed export PNGs and junk

- README: embed resources/Screenshot.png under the intro
- remove accidentally-committed app export outputs (spectrogram_full.png,
  spectrogram_export.png) and gitignore them so they don't return
- remove unused resources/wabbit_alpha.png and stray .qwen/settings.json.orig

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

.gitignore

@@ -406,3 +406,8 @@ Makefile
 *.swo
 *.swn
 /build/compile_commands.json
+
+# rspektrum: PNG exports written by the app (E / --render) into the working dir
+spectrogram_full.png
+spectrogram_export.png
+*.orig