fix(notifications): always serialize streams so clearing the last subscription saves

Turning off the final notification subscription (going from one opted-in
stream to zero) failed with "could not save" and the toggle stuck on. The
backend's PUT /auth/me/notifications/subscriptions validator requires the
`streams` field (body('streams').isArray()), but kotlinx.serialization omits a
property equal to its default (encodeDefaults=false). NotificationSubscriptionsDto
defaulted `streams` to emptyList(), so an empty set serialized to `{}` and the
backend rejected it 400 "Validation failed". Any non-empty set included the
field, so only the last toggle-off broke — regardless of which stream it was.

Remove the default from NotificationSubscriptionsDto.streams so kotlinx always
emits the field; an empty set now sends `{"streams":[]}` (200). The one call
site already passes streams explicitly and the server always returns the field,
so response decoding is unaffected. Add a regression test asserting the empty
DTO serializes to `{"streams":[]}` under the production Json config.

Verified on-device (AVD) against the live site and via the live API
(`{}` -> 400, `{"streams":[]}` -> 200).

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

app/src/main/java/com/runicgateway/app/data/api/dto/NotificationsDto.kt

@@ -60,8 +60,15 @@ data class NotificationStreamsDto(
  * `GET/PUT /auth/me/notifications/subscriptions` — the user's opted-in stream ids.
  * PUT replaces the full set; unknown ids are dropped server-side and the stored set
  * echoed back.
+ *
+ * [streams] intentionally has NO default: this DTO doubles as the PUT body, and the
+ * backend validator requires the `streams` field (`body('streams').isArray()`).
+ * kotlinx omits a property equal to its default (encodeDefaults=false), so a default
+ * of `emptyList()` would drop the field when the user clears their LAST subscription,
+ * sending `{}` → 400 "Validation failed" (the "can't turn off the last one" bug). With
+ * no default the empty list always serializes as `{"streams":[]}`. Do not re-add a default.
  */
 @Serializable
 data class NotificationSubscriptionsDto(
-    val streams: List<String> = emptyList(),
+    val streams: List<String>,
 )

app/src/test/java/com/runicgateway/app/data/api/dto/NotificationsDtoTest.kt

@@ -3,6 +3,7 @@
  */
 package com.runicgateway.app.data.api.dto
 
+import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
@@ -58,6 +59,15 @@ class NotificationsDtoTest {
         assertEquals(listOf("news.post", "champ.start"), dto.streams)
     }
 
+    @Test fun emptySubscriptionsStillSerializeStreamsField() {
+        // Regression: clearing the LAST subscription sends an empty set. The backend
+        // validator requires `streams`, so it must be present as `[]`, not omitted.
+        // Uses the production Json config (no encodeDefaults) to prove the field is
+        // always emitted because the DTO field has no default.
+        val body = json.encodeToString(NotificationSubscriptionsDto(emptyList()))
+        assertEquals("""{"streams":[]}""", body)
+    }
+
     @Test fun settingsPushBlockDecodes() {
         val dto = json.decodeFromString<SettingsDto>(
             """{"site_title":"Shard","brand":{"name":"Shard"},"push":{"ntfyUrl":"https://ntfy.shard.tld"}}""",