docs: trim README to core features only; fix: remove duplicate save button in Android settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/screen/SettingsScreen.kt

@@ -203,9 +203,6 @@ fun SettingsScreen() {
 
         Spacer(Modifier.height(8.dp))
 
-        // ── Save button (BOTTOM) ─────────────────────────────────────────────
-        SaveButton(vm, state.isSaving, state.savedOk, state.error)
-
         HorizontalDivider(modifier = Modifier.padding(vertical = 8.dp), color = NsBorder)
 
         OutlinedButton(