Stage 17 (contour round 3): Telegram Mini Apps polish, board scroll, keyboard overlay

- Telegram (lib/telegram.ts): chrome colours (setHeaderColor/setBackgroundColor/setBottomBarColor) match Telegram's header/bg/bottom bar to the app; native BackButton on sub-screens (app chevron hidden in TG); HapticFeedback on tile place/commit/error; enableClosingConfirmation while a game is open; disableVerticalSwipes so swipe-to-minimise doesn't fight tile drag / board scroll
- #9 board-only vertical scroll: Screen 'column' mode lets the board area scroll while score/status/rack/tab bar stay fixed (zoom keeps its own scroll)
- #10 check-word dialog opens in Modal keyboard-overlay mode (top-anchored, keyboard overlays the empty area) — no resize/relayout jank; other modals stay keyboard-aware
- docs: UI_DESIGN Telegram integration + vertical fit/keyboard; PLAN round 2-3 follow-ups

ui/src/components/Modal.svelte

@@ -4,18 +4,21 @@
   let {
     title = '',
     onclose,
+    overlayKeyboard = false,
     children,
-  }: { title?: string; onclose?: () => void; children?: Snippet } = $props();
+  }: { title?: string; onclose?: () => void; overlayKeyboard?: boolean; children?: Snippet } = $props();
 
   // Track the visual viewport so the backdrop covers only the area above an open
   // mobile keyboard: dvh alone shrinks the sheet but the fixed, layout-viewport
   // backdrop still centres it behind the keyboard. Sizing the backdrop to
   // visualViewport keeps the sheet (and the start of a chat) fully on screen.
+  // overlayKeyboard opts out: the sheet is small and top-anchored, so the keyboard
+  // simply overlays the empty lower area — no resize, no relayout jank (e.g. check word).
   let vh = $state(0);
   let top = $state(0);
   $effect(() => {
     const vv = typeof window !== 'undefined' ? window.visualViewport : null;
-    if (!vv) return;
+    if (!vv || overlayKeyboard) return;
     const update = () => {
       vh = vv.height;
       top = vv.offsetTop;
@@ -34,6 +37,7 @@
 <!-- svelte-ignore a11y_no_static_element_interactions -->
 <div
   class="backdrop"
+  class:overlay={overlayKeyboard}
   style:height={vh ? `${vh}px` : null}
   style:top={vh ? `${top}px` : null}
   onclick={() => onclose?.()}
@@ -61,6 +65,12 @@
     padding: 16px;
     z-index: 40;
   }
+  /* Overlay mode: top-anchor the (small) sheet and don't track the keyboard, so the
+     soft keyboard overlays the empty lower area without resizing/relaying out. */
+  .backdrop.overlay {
+    align-items: flex-start;
+    padding-top: 12vh;
+  }
   .sheet {
     background: var(--surface);
     color: var(--text);