fix the server form's required-field errors

Flag the field that is actually blank instead of one combined "Name and address are
required", and focus it so it scrolls clear of the keyboard. The connection/save status
moves above the buttons in the ime-padded bottom bar, where the keyboard cannot hide it.

app/src/test/kotlin/com/subtracks/ui/settings/AddSourceValidationTest.kt

@@ -0,0 +1,22 @@
+package com.subtracks.ui.settings
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class AddSourceValidationTest {
+    @Test
+    fun flagsOnlyTheMissingFields() {
+        val missingName = AddSourceState(name = "", address = "https://music.example.com").validated()
+        assertTrue("a blank name should be flagged", missingName.nameError)
+        assertFalse("a present address should not be flagged", missingName.addressError)
+
+        val missingAddress = AddSourceState(name = "Home", address = "").validated()
+        assertFalse("a present name should not be flagged", missingAddress.nameError)
+        assertTrue("a blank address should be flagged", missingAddress.addressError)
+
+        val complete = AddSourceState(name = "Home", address = "https://music.example.com").validated()
+        assertFalse(complete.nameError)
+        assertFalse(complete.addressError)
+    }
+}