feat(debug): mark debug builds with a distinct icon, label, and ribbon

Make a debug install unmistakable next to the production app, Flutter-style:

- src/debug resource overrides: "Calendula Debug" label and a burnt-orange
  launcher-icon background (production stays slate). The .debug applicationId
  suffix already lets both install side by side.
- DebugRibbon: a "DEBUG" corner ribbon overlaid in MainActivity, gated on
  BuildConfig.DEBUG so release/releaseTest never show it. Enables the
  buildConfig feature for the flag.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -90,6 +90,8 @@ android {
 
     buildFeatures {
         compose = true
+        // BuildConfig.DEBUG gates the in-app debug ribbon (see DebugRibbon).
+        buildConfig = true
     }
 
     // Don't embed AGP's dependency-metadata block in the APK signing block. It's