fix(ui): own back chevron in Telegram on all platforms; drop the native BackButton

The Telegram native BackButton does not render in the windowed Mini App (the
owner's emulator + a fresh beta TG report backVisible=false on both iOS and
Android), so relying on it lost back navigation — iOS had no back affordance at
all. Show the app's own back chevron whenever there is a back target, on every
platform (Header showBack = !!back), and drop the now-dead native BackButton
effect (App.svelte). The native close control stays — a windowed Mini App
cannot hide it (no Telegram API).

WIP: the temp lobby diagnostic overlay and the requestFullscreen no-op remain
for the owner's emulator test; finalize after confirmation.

ui/src/components/Header.svelte

@@ -1,6 +1,5 @@
 <script lang="ts">
   import { navigate } from '../lib/router.svelte';
-  import { insideTelegram, isTelegramAndroid } from '../lib/telegram';
   import { connection } from '../lib/connection.svelte';
   import { t } from '../lib/i18n/index.svelte';
   import { app } from '../lib/app.svelte';
@@ -9,10 +8,10 @@
 
   let { title, back, grow = false }: { title: string; back?: string; grow?: boolean } = $props();
 
-  // Inside Telegram the native header back button (App.svelte) is the back control, so the app's
-  // own chevron is hidden to avoid two back affordances — except on Android, where the native
-  // BackButton does not render in the (non-fullscreen) presentation, so the app shows its own.
-  const showBack = $derived(!!back && (!insideTelegram() || isTelegramAndroid()));
+  // The app always shows its own back chevron when there is a back target — on every platform, in
+  // and out of Telegram. The native Telegram BackButton is not used: it does not render reliably in
+  // the windowed Mini App (relying on it would lose back navigation there).
+  const showBack = $derived(!!back);
 </script>
 
 <header class="nav" class:grow>