fix(ui): per-bot invite caption + friendly self-redeem note

- The share caption is now in the bot's language: 'Давай играть в Эрудит!' (ru bot),
  "Let's play Scrabble!" (en bot) — picked by the session service language, not the
  interface language.
- Redeeming your own invite (deep link or manual) no longer shows the scary
  'can't do that to yourself' error; it shows a friendly neutral note instead.

ui/src/lib/app.svelte.ts

@@ -493,7 +493,13 @@ async function routeStartParam(param: string): Promise<void> {
         showToast(t('friends.added', { name: friend.displayName }));
         void refreshNotifications();
       } catch (err) {
-        handleError(err);
+        // Tapping your own invite link redeems your own code: show a friendly note, not
+        // the scary "can't do that to yourself" error.
+        if (err instanceof GatewayError && err.code === 'self_relation') {
+          showToast(t('friends.selfInvite'));
+        } else {
+          handleError(err);
+        }
       }
       return;
     default: