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

.qwen/settings.json.orig

@@ -1,7 +0,0 @@
-{
-  "permissions": {
-    "allow": [
-      "Bash(make)"
-    ]
-  }
-}

README.md

@@ -13,6 +13,8 @@ export either the picture (PNG) or the isolated audio (WAV). rspektrum runs thre
 ways: a native desktop app (C + raylib), a headless command-line renderer, and a
 WebAssembly build in the browser.
 
+![rspektrum spectrogram view with mLnL annotation overlay](resources/Screenshot.png)
+
 ---
 
 ## What it's for