UI: enable the back-swipe inside Telegram (fix lost swipe-back)

The gesture was disabled inside Telegram on the mistaken assumption it would
fight a Telegram horizontal close gesture — but Telegram's only close/minimise
gesture is vertical (disableVerticalSwipes, Bot API 7.7+). Disabling it there,
together with dropping the old left-edge swipe, removed every swipe-back in
Telegram (which that edge-swipe — deliberately enabled in TG, commit 7e34897 —
had provided).

Make shouldArm Telegram-agnostic (drop the insideTelegram guard + its now-unused
import); the gesture works in Telegram too, alongside the native BackButton.
Drop the redundant overscroll-behavior-x on .router (body already sets
overscroll-behavior: none). Regression: an e2e drives the swipe under an
injected Telegram WebApp stub and asserts it navigates back.

docs/UI_DESIGN.md

@@ -31,8 +31,9 @@ Login uses `Screen`.
   back). The live parent mounts only on commit, which suppresses that one route slide so the
   hand-off is seamless. The gesture yields to native **vertical scroll** on a vertical move
   and skips the rack, a draggable pending tile, text inputs and the board **while zoomed-in**
-  (so it never fights the board pan); it stays disabled inside Telegram, whose native
-  BackButton owns this.
+  (so it never fights the board pan). It works **inside Telegram** too — Telegram's only
+  close/minimise gesture is *vertical* (disabled separately via `disableVerticalSwipes`), so
+  a horizontal back-swipe has nothing to fight; the native header BackButton complements it.
 - **No hamburger**: navigation is entirely bottom tab bars (the former `Menu.svelte`
   dropdown is gone — it fought the Telegram-fullscreen layout, where it had to be re-centred).
   Destinations beyond the lobby live behind **hub screens** reached from a tab: a ⚙️