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/src/debug/res/values/colors.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Debug-only launcher-icon background. Production is slate (#5C6B7A); the
+    debug build paints the adaptive-icon background burnt orange instead, so the
+    debug icon reads at a glance as "not the real app" on the home screen. The
+    off-white foreground mark contrasts on both. See drawable/ic_launcher_background.
+-->
+<resources>
+    <color name="ic_launcher_background">#FFB23B00</color>
+</resources>

app/src/debug/res/values/strings.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Debug-build resource overrides. Merged on top of src/main for the `debug`
+    build type only (release/releaseTest keep the production values), so the
+    debug app is unmistakable on the launcher: its own label, alongside the
+    real app thanks to the `.debug` applicationId suffix.
+-->
+<resources>
+    <string name="app_name">Calendula Debug</string>
+</resources>