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/src/main/java/com/example/helios_alarm_clock/service/KtorService.kt

@@ -54,7 +54,13 @@ class KtorService : Service() {
         super.onCreate()
         acquireWakeLock()
         startForeground(NOTIFICATION_ID, buildNotification())
-        startServer()
+        try {
+            startServer()
+        } catch (e: Exception) {
+            releaseWakeLock()
+            stopSelf()
+            return
+        }
         rescheduleAlarms()
     }
 
@@ -111,6 +117,13 @@ class KtorService : Service() {
                 post("/set") {
                     try {
                         val req = call.receive<SetAlarmRequest>()
+                        if (req.hour !in 0..23 || req.minute !in 0..59) {
+                            call.respond(
+                                HttpStatusCode.BadRequest,
+                                ErrorResponse("hour must be 0-23, minute must be 0-59")
+                            )
+                            return@post
+                        }
                         val id = UUID.randomUUID().toString()
 
                         val now = Calendar.getInstance()

app/src/main/res/values/colors.xml

@@ -1,10 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <color name="purple_200">#FFBB86FC</color>
-    <color name="purple_500">#FF6200EE</color>
-    <color name="purple_700">#FF3700B3</color>
-    <color name="teal_200">#FF03DAC5</color>
-    <color name="teal_700">#FF018786</color>
     <color name="black">#FF000000</color>
     <color name="white">#FFFFFFFF</color>
-</resources>
+</resources>