Fix START button being disabled before permissions are granted

The button was gated on `granted || running`, but the only thing that
triggers the permission request is tapping the button — catch-22 that
left first-time users with no way to grant permissions.

Always enable the button when not running; the onStartStop handler already
routes correctly (start scanning if granted, else launch the permission
request flow). Updated the helper text to point at this directly.

app/src/main/kotlin/org/soulstone/overwatch/MainActivity.kt

@@ -70,8 +70,8 @@ class MainActivity : ComponentActivity() {
                             threat = threat,
                             score = maxScore,
                             events = events,
-                            canStart = granted || running,
+                            canStart = true,
-                            permissionMessage = if (!granted) "Bluetooth, WiFi + location permissions required" else null,
+                            permissionMessage = if (!granted) "Tap START to grant Bluetooth, WiFi + location permissions" else null,
                             onStartStop = {
                                 if (running) {
                                     DetectionService.stop(this)