Production cleanup: enable R8, add ProGuard rules, validate API input

- Enable R8 minification and resource shrinking for release builds
- Add ProGuard keep rules for Ktor, kotlinx.serialization, Room
- Validate hour/minute range in POST /set endpoint
- Guard wake lock release on server start failure
- Remove unused template colors from colors.xml
- Rewrite README with curl examples, security note, troubleshooting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/proguard-rules.pro

@@ -1,21 +1,27 @@
-# Add project specific ProGuard rules here.
-# You can control the set of applied configuration files using the
-# proguardFiles setting in build.gradle.
-#
-# For more details, see
-#   http://developer.android.com/guide/developing/tools/proguard.html
+# Ktor — keep CIO engine and routing
+-keep class io.ktor.** { *; }
+-keepclassmembers class io.ktor.** { *; }
+-dontwarn io.ktor.**
 
-# If your project uses WebView with JS, uncomment the following
-# and specify the fully qualified class name to the JavaScript interface
-# class:
-#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
-#   public *;
-#}
+# kotlinx.serialization — keep @Serializable classes
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.AnnotationsKt
+-keepclassmembers @kotlinx.serialization.Serializable class ** {
+    *** Companion;
+}
+-keepclasseswithmembers class ** {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+-keep,includedescriptorclasses class com.example.helios_alarm_clock.**$$serializer { *; }
+-keepclassmembers class com.example.helios_alarm_clock.** {
+    *** Companion;
+}
+-keepclasseswithmembers class com.example.helios_alarm_clock.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}
 
-# Uncomment this to preserve the line number information for
-# debugging stack traces.
-#-keepattributes SourceFile,LineNumberTable
+# Room — keep entities
+-keep class com.example.helios_alarm_clock.data.AlarmEntity { *; }
 
-# If you keep the line number information, uncomment this to
-# hide the original source file name.
-#-renamesourcefileattribute SourceFile
+# SLF4J (Ktor dependency) — suppress missing impl warnings
+-dontwarn org.slf4j.**