docs: bake Telegram invite & launch refinements into ARCHITECTURE/UI_DESIGN

- UI_DESIGN: mobile immersive fullscreen on launch (desktop keeps full-size); the
  close confirmation is now mobile-only; friend-code share-via-Telegram deep-link
  (per-bot, by service language), the friendly self-redeem note, and the
  outdated-link lobby notice.
- ARCHITECTURE: the service language rides the Session wire so the client builds
  the per-bot invite link; the friend code is shared as a startapp deep-link with
  graceful spent/expired handling.

docs/UI_DESIGN.md

@@ -67,8 +67,12 @@ Login uses `Screen`.
   theme switcher is hidden; the nav bar takes Telegram's background and `setHeaderColor` /
   `setBackgroundColor` / `setBottomBarColor` paint Telegram's own chrome to match; the
   native header **BackButton** drives back-navigation (the app's chevron is hidden in
-  Telegram); **HapticFeedback** fires on tile placement / commit / error; **closing
-  confirmation** is enabled while a game is open; **vertical swipes** (swipe-to-minimise)
+  Telegram); **HapticFeedback** fires on tile placement / commit / error; on **mobile**
+  clients the app enters **immersive fullscreen** on launch (`requestFullscreen`, Bot API
+  8.0+) like Telegram's own Mini Apps, while desktop keeps the bot's full-size window;
+  **closing confirmation** is enabled while a game is open **on mobile only** (on desktop
+  closing is deliberate and the "changes may not be saved" dialog is just noise — move drafts
+  auto-save); **vertical swipes** (swipe-to-minimise)
   are disabled so they don't fight tile drag or the board scroll; and a live stream dropped
   by a background suspend reconnects silently on return — the connection banner is
   suppressed while hidden and for a short grace after resume (visibilitychange +
@@ -242,8 +246,14 @@ IV 🏅; active games show Your move 🟢 / Opponent's move ⏳; invitations use
   every field is valid. Interface language stays in **Settings** (it writes through to
   the account for durable users).
 - **Friend code**: the issued code sits next to a 📋 copy control; tapping the code or
-  the icon copies it. Flex text inputs carry `min-width:0` so they shrink instead of
-  overflowing in Safari.
+  the icon copies it. **Share via Telegram** wraps the code in a `t.me/<bot>/<app>?startapp=`
+  deep-link and opens Telegram's native share-to-chat sheet (Web Share / clipboard fallback
+  outside Telegram); the link and its caption are for the **same bot the player signed in
+  through** (its service language). Redeeming your **own** invite shows a friendly note, not an
+  error; opening an **outdated** invite link (a used or expired single-use code) lands in the
+  lobby with a calm modal pointing at the right bot (`@<username>`), not a red error on the
+  Friends tab. Flex text inputs carry `min-width:0` so they shrink instead of overflowing in
+  Safari.
 - **History / GCG**: the in-game slide-down history lays each move out in a per-seat grid
   (the word(s) and the move score, no running total); *Export GCG* (the 📤 in the history
   header) shares or downloads the `.gcg` file and appears only once the game is finished.