fix(commons): tighter Base64Image contract + pin serializer wire format

Two follow-up cleanups from the audit.

Base64Image.parse: when the regex matched but the data capture group
was missing, the migrated version returned an empty ByteArray. The
original threw NPE (java.util.Base64.getDecoder().decode(null)). Both
behaviors are accidents — restore the intended contract: throw the
existing "Unable to convert base64 to image" Exception explicitly.

FeedDefinitionSerializerTest gains a serializesToExpectedWireFormat
test that pins the byte-exact JSON output for a representative
multi-field feed. The legacy-Jackson migration claimed byte-identity
but only round-trip and reverse-compat were covered. Any future change
to field ordering / null handling / number formatting now fails this
test loudly, protecting users who have saved feeds on disk and any
downstream consumer expecting the stable order.

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/richtext/Base64Image.kt

@@ -30,12 +30,10 @@ object Base64Image {
 
     @OptIn(ExperimentalEncodingApi::class)
     fun parse(content: String): ByteArray {
-        val matcher = pattern.find(content)
-        if (matcher != null) {
-            val base64String = matcher.groups[2]?.value ?: ""
-            return Base64.decode(base64String)
-        }
-
-        throw Exception("Unable to convert base64 to image $content")
+        val match = pattern.find(content) ?: throw Exception("Unable to convert base64 to image $content")
+        val base64String =
+            match.groups[2]?.value
+                ?: throw Exception("Unable to convert base64 to image $content")
+        return Base64.decode(base64String)
     }
 }

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/feeds/custom/FeedDefinitionSerializerTest.kt

@@ -212,6 +212,44 @@ class FeedDefinitionSerializerTest {
         assertEquals(emptyList(), FeedDefinitionSerializer.deserializeList("  "))
     }
 
+    @Test
+    fun serializesToExpectedWireFormat() {
+        // Pins the byte-exact JSON output so any future change to the
+        // serializer (field order, null handling, number formatting) is
+        // caught here. Users have feeds saved on disk in this exact format
+        // and may share JSON between app versions; downstream consumers
+        // depend on the field order being stable.
+        val feed =
+            FeedDefinition(
+                id = "test-1",
+                name = "Bitcoin",
+                emoji = "₿",
+                pinned = true,
+                pinOrder = 0,
+                source =
+                    FeedSource.Filter(
+                        hashtags = persistentListOf("bitcoin", "btc"),
+                        authors = persistentListOf("abc123"),
+                        relays = persistentListOf("wss://relay.damus.io"),
+                        excludeAuthors = persistentListOf("spammer"),
+                        excludeKeywords = persistentListOf("scam"),
+                        kinds = persistentListOf(1, 6),
+                    ),
+                refreshMode = RefreshMode.LIVE_STREAM,
+                createdAt = 1000L,
+            )
+
+        val expected =
+            """[{"id":"test-1","name":"Bitcoin","emoji":"₿","pinned":true,""" +
+                """"pinOrder":0,"refreshMode":"LIVE_STREAM","createdAt":1000,""" +
+                """"source":{"type":"filter","hashtags":["bitcoin","btc"],""" +
+                """"authors":["abc123"],"relays":["wss://relay.damus.io"],""" +
+                """"excludeAuthors":["spammer"],"excludeKeywords":["scam"],""" +
+                """"kinds":[1,6]}}]"""
+
+        assertEquals(expected, FeedDefinitionSerializer.serializeList(listOf(feed)))
+    }
+
     @Test
     fun parsesLegacyJacksonOutput() {
         // Wire format previously emitted by ObjectMapper. Users have feed