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>

app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt

@@ -8,6 +8,7 @@ import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.appcompat.app.AppCompatActivity
 import androidx.compose.foundation.isSystemInDarkTheme
+import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.getValue
@@ -29,6 +30,7 @@ import de.jeanlucmakiola.calendula.ui.common.LocalShowHourLines
 import de.jeanlucmakiola.calendula.ui.common.LocalUse24HourFormat
 import de.jeanlucmakiola.calendula.ui.WidgetNavRequest
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
+import de.jeanlucmakiola.calendula.ui.common.DebugRibbon
 import de.jeanlucmakiola.calendula.ui.crash.CrashReportActivity
 import de.jeanlucmakiola.calendula.ui.crash.CrashReportDialog
 import de.jeanlucmakiola.calendula.ui.crash.submitCrashReport
@@ -101,19 +103,24 @@ class MainActivity : AppCompatActivity() {
                 darkTheme = darkTheme,
                 dynamicColor = settings.dynamicColor,
             ) {
-                CompositionLocalProvider(
-                    LocalUse24HourFormat provides use24Hour,
-                    LocalShowHourLines provides settings.showHourLines,
-                ) {
-                    RootScreen(
-                        modifier = Modifier.fillMaxSize(),
-                        requestedDetailKey = requestedDetailKey,
-                        onDetailKeyConsumed = { requestedDetailKey = null },
-                        widgetNavRequest = requestedNav,
-                        onWidgetNavConsumed = { requestedNav = null },
-                        requestedImportUri = requestedImportUri,
-                        onImportConsumed = { requestedImportUri = null },
-                    )
+                Box(modifier = Modifier.fillMaxSize()) {
+                    CompositionLocalProvider(
+                        LocalUse24HourFormat provides use24Hour,
+                        LocalShowHourLines provides settings.showHourLines,
+                    ) {
+                        RootScreen(
+                            modifier = Modifier.fillMaxSize(),
+                            requestedDetailKey = requestedDetailKey,
+                            onDetailKeyConsumed = { requestedDetailKey = null },
+                            widgetNavRequest = requestedNav,
+                            onWidgetNavConsumed = { requestedNav = null },
+                            requestedImportUri = requestedImportUri,
+                            onImportConsumed = { requestedImportUri = null },
+                        )
+                    }
+                    // A persistent corner marker so a debug build is never
+                    // mistaken for the production app; compiled out of release.
+                    if (BuildConfig.DEBUG) DebugRibbon()
                 }
                 pendingCrashReport?.let { report ->
                     CrashReportDialog(

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/DebugRibbon.kt

@@ -0,0 +1,50 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.BoxScope
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.rotate
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.compose.ui.zIndex
+
+/**
+ * A Flutter-style "DEBUG" corner ribbon, drawn across the top-right corner of
+ * the app. Deliberately a stark, un-themed marker (not a product component) so a
+ * debug build is unmistakable at a glance — gate it on `BuildConfig.DEBUG` at the
+ * call site so it never reaches a release build. Non-interactive: it's a plain
+ * label with no pointer handler, so taps fall through to whatever is beneath it.
+ *
+ * Drop it in as the last child of a full-screen [androidx.compose.foundation.layout.Box]
+ * so it overlays the UI.
+ */
+@Composable
+fun BoxScope.DebugRibbon() {
+    Text(
+        text = "DEBUG",
+        color = Color.White,
+        fontSize = 10.sp,
+        fontWeight = FontWeight.Bold,
+        letterSpacing = 1.sp,
+        textAlign = TextAlign.Center,
+        modifier = Modifier
+            .align(Alignment.TopEnd)
+            .zIndex(1f)
+            // Push the band out so its midline crosses the very corner, then
+            // rotate it to the classic 45° ribbon.
+            .offset(x = 36.dp, y = 24.dp)
+            .rotate(45f)
+            .background(Color(0xFFB23B00))
+            .width(140.dp)
+            .padding(vertical = 2.dp),
+    )
+}