Stage 7 polish (round 2): layout/zoom/tab-bar/hint/check fixes

- nav bar grows ONLY in game (other screens: minimal nav, content fills); tab bar always bottom
- tab bar: tighter icon/label spacing, bigger icons, hint badge on the icon corner
- board zoom reworked to width-based (real native scroll, fixes Safari/Chrome) + constant cqw labels; pinch & swipe-to-history dropped (conflict), double-tap kept, history via menu
- beginner bonus labels shrunk to fit cells
- Draw opens exchange directly (no confirm); confirm popovers restyled like the hamburger dropdown (vertical); removed the floating direction toggle
- pending tiles darker bg (no outline); last-word dark-tile highlight (static / 1s flash)
- check button disabled for <2/>15 chars, already-checked, or 5s cooldown
- global user-select:none (inputs exempt); docs updated; TODO-4 alphabet-on-wire

ui/src/components/Screen.svelte

@@ -13,6 +13,7 @@
     tabbar,
     children,
     scroll = true,
+    growNav = false,
   }: {
     title: string;
     back?: string;
@@ -20,13 +21,14 @@
     tabbar?: Snippet;
     children?: Snippet;
     scroll?: boolean;
+    growNav?: boolean;
   } = $props();
 </script>
 
 <div class="screen">
-  <Header {title} {back} {menu} />
+  <Header {title} {back} {menu} grow={growNav} />
   <AdBanner />
-  <main class="content" class:scroll>{@render children?.()}</main>
+  <main class="content" class:scroll class:fill={!growNav}>{@render children?.()}</main>
   {#if tabbar}
     <nav class="tabbar">{@render tabbar()}</nav>
   {/if}
@@ -42,6 +44,9 @@
     flex: 0 1 auto;
     min-height: 0;
   }
+  .content.fill {
+    flex: 1 1 auto;
+  }
   .content.scroll {
     overflow-y: auto;
   }