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/HoldConfirm.svelte

@@ -96,7 +96,14 @@
     transform: translateX(-50%);
     z-index: 19;
     display: flex;
-    gap: 4px;
+    flex-direction: column;
+    gap: 2px;
     white-space: nowrap;
+    background: var(--surface);
+    border: 1px solid var(--border);
+    border-radius: var(--radius-sm);
+    box-shadow: var(--shadow);
+    padding: 4px;
+    min-width: 132px;
   }
 </style>